The coronation of King Charles III and Queen Camilla

The leader of Britain's largest anti-monarchy group was among demonstrators arrested by police near the coronation procession route, according to protesters.

Graham Smith was detained on Saturday morning, a member of the Republic group’s parent organization, the Alliance of European Republican Movements (AERM), said in a video shared on social media. 

The Metropolitan Police tweeted: "Earlier today we arrested four people in the area of St Martin’s Lane. They were held on suspicion of conspiracy to cause public nuisance."

A further three people were arrested "on suspicion of possessing articles to cause criminal damage," the force added. And "a number of arrests" have been made of people suspected of breaching the peace.

Republic had said earlier on Twitter that police "won't say" why their demonstrators were detained. "So much for the right to peaceful protest," the group said.

The group claimed it was expecting between 1,500 and 2,000 people to join the group at its protest in Trafalgar Square, just south of the royal procession route.

"Instead of a coronation we want an election. Instead of Charles we want a choice. It’s that simple," the group tweeted on Saturday.

The Metropolitan Police has been scrutinized for its tough approach toward protests around the coronation.

"Our tolerance for any disruption, whether through protest or otherwise, will be low," the force wrote on Twitter this week. "We will deal robustly with anyone intent on undermining this celebration."

Two hours of pre-coronation music have begun with a performance of a trio of Bach choruses by the Monteverdi Choir and the English Baroque soloists.

Music underpins the entire coronation – every stage is marked by either a grand choral work, an ethereal motet, an extravagant organ composition or an evocative melody, all performed by some of the most accomplished singers and musicians in the world. Many of the pieces that are now staples of the English choral repertoire originated as commissions for the coronation of a monarch.

For this coronation, Buckingham Palace has been keen to stress King Charles’ involvement in the musical choices, in particular the new compositions. And in many ways, Charles has played it safe. The Aberdeen-based Welsh composer Paul Mealor has written “Coronation Kyrie,” the first Welsh-language work at a coronation, which will be performed by the Welsh bass-baritone Bryn Terfel. Mealor has had a long association with the royal family – he was brought to national attention when Prince William and Kate Middleton chose his motet "Ubi Caritas" to be sung at their wedding in 2011. The British composer Roxanna Panufnik – whose prolific output includes choral works, opera, ballet and film scores – has written “Coronation Sanctus,” which is likely to be a highlight.

But eyebrows were most certainly raised when it was announced that Charles had tapped Andrew Lloyd Webber – whose music is more familiar to audiences in the West End than congregations in Westminster Abbey – to write a Coronation Anthem, “Make a Joyful Noise.” Purists may wrinkle their noses, but one thing we can be sure of, it will be performed to the highest standard by the Choir of Westminster Abbey, directed by the Organist and Master of the Choristers, Andrew Nethsingha.

Other new works to watch out for include a short overture composed by Judith Weir, Master of The King’s Music; “Agnus Dei” by British-America composer Tarik O’Regan, and the first appearance by a gospel choir at a coronation. The Ascension Choir will join the Westminster Abbey Choir for “Alleluia (O Clap your Hands)” and "Alleluia (O Sing Praises)," a two-part commission from the well-known TV and film composer Debbie Wiseman.

Whatever the Palace would like us to think, Charles has by no means ditched the traditional elements of the coronation. We’ll hear the Anglican choral staple, “I was glad,” by Hubert Parry, first performed at the coronation of Edward VII in 1902 and wheeled out for every coronation since (although Parry rewrote it a couple of times until it settled on the version we know and love today). Watch out for the “vivats” in the middle – acclamation to the monarch sung by the King’s Scholars of Westminster School, and which are rewritten for every coronation. (Vivat Regina Camilla! Vivat Rex Carolus! – Long Live Queen Camilla! Long Live King Charles!) And there is pride of place, too, for "Zadok the Priest," composed by George Fredrick Handel for the crowning of King George II in 1727, and – remarkably – performed just before the anointing of the monarch at every coronation since.

Crowds have had their first sighting of King Charles III today, as the monarch and his wife, Camilla, were driven up the Mall to Buckingham Palace ahead of his coronation.

Charles waved to well-wishers lining the street as the motorcade proceeded.

In less than two hours he will arrive at Westminster Abbey for the ceremony.

WATCH WHEN IT HAPPENED:

Katy Perry and actress Maggie Smith have arrived at Westminster Abbey, joining a plethora of well-known individuals filing into the venue.

Not to be overshadowed by their A-list peers, a number of senior political figures have also been arriving.

Who else has been spotted entering the Abbey:

Singers Lionel Richie and Nick Cave, and British actress Emma Thompson, are among the first guests to filter into Westminster Abbey as King Charles' coronation ceremony nears.

WATCH HERE:

Actresses Judi Dench and Joanna Lumley, and composer Andrew Lloyd Webber, have also arrived.

Around 2,300 people were sent invitations to the Westminster Abbey service, according to Earl Marshall, the Duke of Norfolk, who was involved in the preparations.

Who's there (and who's not): While Buckingham Palace doesn’t release a detailed guest list, it has confirmed that the congregation will be made up of members of the royal family as well as international representatives from 203 countries, Nobel Prize winners and members of the British government, alongside community and charity volunteers.

Prince Harry has confirmed his attendance but his wife, Meghan, will be staying in California with their two children, Prince Archie and Princess Lilibet. It is understood Meghan opted to forgo the celebrations as the day coincides with Archie’s 4th birthday.

US President Joe Biden also won’t be attending and instead first lady Jill Biden will lead the American delegation. We'll undoubtedly see allies of key nations in attendance, with French President Emmanuel Macron, Polish President Andrzej Duda and Australian Prime Minister Anthony Albanese among those who have publicly confirmed they'll be attending. 

We can also expect to see a number of royals from around the world make their way to London, like Spain’s King Felipe and Queen Letizia and King Philippe and Queen Mathilde of Belgium.

More than 11,500 police officers are being deployed in London on Saturday, according to the London Metropolitan Police Service, and there are plans for facial recognition technology to be used in the capital.

It is the largest one-day deployment in decades, London's Metropolitan Police Deputy Assistant Commissioner Ade Adelekan said Wednesday.

These officers will be part of more than 29,000 deployments in the lead-up to May 6 over the rest of the weekend, according to a statement from the Met.

The Met said the operation -- labelled Golden Orb -- will see officers line the processional route, manage crowds and road closures, protect high-profile individuals and carry out searches with specialist teams.

There are also plans for facial recognition technology to be used in central London.

"Our tolerance for any disruption, whether through protest or otherwise, will be low. We will deal robustly with anyone intent on undermining this celebration," the Met added.

Facial recognition concerns: Campaign groups have criticized the use of live facial recognition technology during the coronation.

It is expected to be the largest-ever use of the technology in Britain, with hundreds of thousands of people expected to line the streets on Saturday.

Privacy campaign group Big Brother Watch called it “an authoritarian mass surveillance tool that turns the public into walking ID cards," describing the technology as "Orwellian."

“We all have the right to go about our lives without being watched and monitored, but everyone at the coronation is at risk of having their faces scanned by oppressive facial recognition technology,” Emmanuelle Andrews of human rights group Liberty said on Twitter.

Coronation Day is here and festivities are already off to a damp start thanks to showers that moved through London on Friday, with even more rain expected for the big day.

“A further area of rain is expected to move into southwest England early on Saturday, moving east and north through the day with some heavy bursts at times. This is likely to bring some rain to London by around lunchtime," said UK Met Office deputy chief forecaster, Steven Keates.

One bit of good news: Temperatures will be at or slightly above normal for early May.

The high temperature for Saturday in London was initially predicted to be between 16-18°C (60-64°F). By Saturday, the UK Weather office has also downgraded expected temperatures to between 12-14°C (53-57°F). For the coronation of George V, temperatures reached 17°C (63°F) back on June 22, 1911.

Unfortunately, it's also possible this could be one of the soggiest coronations in more than a century. For the coronation of George VI, zero sunshine hours were reported on May 12, 1937 along with 8 millimeters (0.32 inches) of rain.

While it is not forecast to be a complete washout for Charles' coronation on Saturday, the highest rain chances do happen to coincide with several outdoor scheduled activities.

Rain chances will start off low in the morning, but are expected to peak midday, while the King and Queen Consort are in Westminster Abbey, as well as when they are heading to the Buckingham Palace, showers are likely to salute them the entire way.

With celebrations continuing through the Bank Holiday, Sunday will likely be drier across many regions, but still some low end rain chances with cloudy skies. On Monday, heavy rain, and possibly a few thunderstorms return across parts of central, eastern and southern England, with parts of northeast England and Scotland looking gloomy and wet as well.

"Eastern areas will start out mostly dry on Monday but a band of rain in the west will sweep eastwards throughout the day bringing rain to many places," the UK Met office said in their forecast. "This rain will be followed by sunshine and scattered showers. Winds are expected to be fresh to strong with temperatures near average."

CNN's Taylor Ward contributed to this post. This post has been updated.

It's the question everyone's been asking: Who managed to get a coveted invite?

According to Earl Marshall, the Duke of Norfolk -- the man that orchestrated the entire occasion (in consultation with the UK government, church and many other parties) -- around 2,300 people were sent invitations to the Westminster Abbey service.

While Buckingham Palace doesn’t release a detailed guest list, it has confirmed that the congregation will be made up of members of the royal family as well as international representatives from 203 countries, Nobel Prize winners and members of the British government, alongside community and charity volunteers.

Prince Harry has confirmed his attendance but his wife, Meghan, will be staying in California with their two children, Prince Archie and Princess Lilibet. It is understood Meghan opted to forgo the celebrations as the day coincides with Archie’s fourth birthday.

US President Joe Biden also won’t be attending and instead first lady Jill Biden will lead the American delegation. We'll undoubtedly see allies of key nations in attendance, with French President Emmanuel Macron, Polish President Andrzej Duda and Australian Prime Minister Anthony Albanese among those who have publicly confirmed they'll be attending.

We can also expect to see a number of royals from around the world make their way to London, like Spain’s King Felipe and Queen Letizia and King Philippe and Queen Mathilde of Belgium.

Additionally, 400 young people from charities selected by the King and Queen have been invited to watch the service from nearby St. Margaret’s Church, while military veterans, healthcare workers and charity representatives have been given spaces to watch on the processional route and in special stands put up along The Mall and near Buckingham Palace.

But it is still a much smaller guest list than the one drawn up for Queen Elizabeth II’s coronation in 1953, when temporary structures had to be erected within the abbey to accommodate the more than 8,000 people invited.

WATCH:

The route King Charles and Queen Camilla will travel today is significantly shorter than the Queen’s five-mile journey to Westminster Abbey back in 1953. Take a look: