The assailant drives a vehicle over Westminster Bridge into a crowd, injuring dozens, including three police officers.
The car drives on and crashes into perimeter railings surrounding the Houses of Parliament.
Attacker runs around the corner, approximately 50 meters, then through Parliament's gates, where he stabs an unarmed policeman.
Police open fire on attacker, who is shot dead.
London Ambulance Service receives reports of an incident on Westminster Bridge.
2:46 p.m. - Emergency response
First ambulance crew arrives at the scene. Prime Minister Theresa May is evacuated from Parliament around the same time.
2:47 p.m. - House of Commons is informed
The sitting House of Commons is suspended and Members of Parliament are also placed in lockdown.
David Lidington, Leader of the House of Commons, tells MPs a police officer was stabbed and the assailant appeared to have been shot. He adds that an air ambulance is removing casualties from scene.
MPs are told to remain in the chamber and stay away from windows.
3:00 p.m. - Full lockdown
Westminster is put on lockdown by police, including roads, Parliament buildings; the Westminster Underground station is also secured. TV images emerge of a car crashed into the railings.
3:32 p.m. - 'Terrorist incident'
London Metropolitan Police calls the attack a "terrorist incident."
Authorities tell public to avoid the expanded area around Parliament. The gates of Buckingham Palace are closed as well as Downing Street and Horse Guards Parade.
The London Eye Ferris wheel, a sightseeing destination on the South Bank, is suspended with tourists still inside the capsules.
5:19 p.m. - Woman pulled from Thames
A woman is pulled from the River Thames alive but with injuries, she was found near Westminster Bridge, where the attack occurred, a Port of London Authority spokesman tells CNN.
5:50 p.m. - Three deaths confirmed
Three people have died, Britain's most senior counter-terrorism police officer Mark Rowley announces in a news conference. He confirms the attacker was also killed.
"Our strength as a city is our ability to stand together at such terrible times," Rowley says. "My thoughts are with all those who have been affected by today's attack -- and as a service we have lost one of our own as he acted to protect the public and his colleagues.
"This is a day that we had planned for -- that we all hoped would never happen -- but sadly it is now a reality."
5:50 p.m. - Parliament evacuated
Hundreds of people are evacuated from Parliament to nearby Westminster Abbey, Britain's Press Association news agency reports. A Metropolitan Police superintendent says it will take around 90 minutes to process everyone. Authorities add that people will be assessed to see if they have any information.
6:23 p.m. - Parliament defiant
UK Parliament tweets that both the House of Commons and House of Lords will open and sit as usual on Thursday despite the attack.
7:25 p.m. - Lockdown lifted
MP Stephen McPartland tweets: "We have just been released from Parliament." The House of Commons later confirms to CNN that the Palace of Westminster is no longer in lockdown.
8:53 p.m. - UK PM speaks
Prime Minister Theresa May condemns "sick and depraved terrorist attack," in a briefing. She makes her remarks after meeting with COBRA, the UK's emergency cabinet committee.
10:29 p.m. - Slain cop identified
Mark Rowley announces that the death toll has risen to four
, including one police officer, and 40 were injured in the attack. He later revises the death toll back to three.
He announces the officer killed was Keith Palmer
, 48, a husband and father and a 15-year veteran.
At Scotland Yard, headquarters of London's police force, flags were flying at half-staff on Wednesday evening, in honor of Palmer and the other victims.
THURSDAY 6:03 a.m. - Seven arrested in police raids
Police raided an apartment in Birmingham and other locations overnight, Britain's Press Association reports early on Thursday morning.
Seven people were arrested at six addresses following enquiries in London and Birmingham, acting deputy police commissioner Mark Rowley tells journalists. Rowley adds that police believe the attacker acted alone and was inspired by international terrorism.
9:26 a.m. - First victim identified
Aysha Frade is identified as one of the victims killed in the attack near Parliament Square, a spokesperson for the Foreign Ministry in Spain tells CNN.
The 43-year-old British teacher with Spanish origins, had been living in London for several years with her Portuguese husband, according to Ramon Garcia Vasquez, the mayor of the Spanish town of Betanzos.
2:08 p.m. - Utah man also died in attack
American tourist Kurt Cochran is identified by family and his church as the second victims who died in the London attack. Cochran was in London celebrating his 25th wedding anniversary with his wife Melissa.
His wife suffered a broken leg and rib as well as a cut to her head but she is expected to recover from her injuries, her brother Clint wrote on social media.
12:39 p.m. - ISIS claims responsibility
ISIS claims responsibility for Wednesday's Westminster attack but does not offer evidence of any connections to the assailant. ISIS-affiliated news agency, Amaq, reports that the attacker was a "soldier of Islamic State."
CNN Terrorism analyst Paul Cruickshank said the language used by ISIS doesn't necessarily mean that the group is claiming it had connections to the attacker. He says this type of language has been used in the past by the group for attacks it believes it helped inspire.
3:33 p.m. - Police name attacker
London Metropolitan Police formally identify the perpetrator as 52-year-old Khalid Masood.
A statement says Masood, who was born in Kent, a county in the south-east, had previous convictions for violence. It adds that he is thought to have been living in the West Midlands in central England.
He has never been convicted of any terrorism offenses, according to the police statement. Speaking before Masood was named, Prime Minster Theresa May said the attacker was known to authorities for links to violent extremism.
He was investigated "some years ago" by security services, but was regarded as a "peripheral figure," May said in the House of Commons.
9.17 p.m. - Death toll rises to 4
A man who was being treated at a local hospital has become the fourth fatality of the rampage, according to London Metropolitan Police.
The 75-year-old man had been receiving medical treatment following the attack and his life support was withdrawn this evening, said police.