Tech companies including Facebook, Twitter and YouTube are responding to the attacks, one of which appeared to be live-streamed by one of the shooters.
The disturbing video, which has not been verified by CNN, purportedly shows a gunman walking into a mosque and opening fire.
Twitter has a "dedicated team in place for managing exigent and emergency situations such as this," the company said in a statement to CNN.
Facebook is "removing any praise or support for the crime and the shooter or shooters as soon as we're aware," a spokesperson said.
Google and YouTube added in a statement: "Our hearts go out to the victims of this terrible tragedy. Shocking, violent and graphic content has no place on our platforms, and is removed as soon as we become aware of it. As with any major tragedy, we will work cooperatively with the authorities.”
But some experts have criticized the platforms for their response to other harmful content.
"While Google, YouTube, Facebook and Twitter all say that they're cooperating and acting in the best interest of citizens to remove this content, they're actually not because they're allowing these videos to reappear all the time," Lucinda Creighton, a senior adviser at international policy organization the Counter Extremism Project, told CNN Business.
Read more about the companies' responses here.
France is joining countries including the UK and the US in confirming that mosques and places of worship will see increased security on Friday.
Interior Minister Christophe Castaner said he had asked local officials to "strengthen surveillance" at places of worship as a precaution. "Patrols will be provided in the vicinity of denominational spaces," he wrote on Twitter.
Officials in London and police in Los Angeles, New York and Minneapolis earlier announced that they would be stepping up security at mosques.
Queen Elizabeth II, who is New Zealand's monarch and the head of the Commonwealth, has released a message to the people of the country.
She said she was "deeply saddened" by the news, and paid tribute to emergency services.
Here's the full message:
“I have been deeply saddened by the appalling events in Christchurch today. Prince Philip and I send our condolences to the families and friends of those who have lost their lives.
I also pay tribute to the emergency services and volunteers who are providing support to those who have been injured.
At this tragic time, my thoughts and prayers are with all New Zealanders.
Yousef bin Ahmad Al-Othaimeen, the Secretary General of the Organization of Islamic Cooperation (OIC), has condemned the attacks. The OIC represents 57 nations with large or majority-Muslim populations.
"The brutal crime had shocked and hurt the feelings of all Muslims around the world, and served as a further warning on the obvious dangers of hate, intolerance, and Islamophobia," he said. Al-Othaimeen urged the New Zealand authorities to investigate the attack thoroughly.
Journalist Chris Lynch, host of the radio show "Canterbury Mornings" on Newstalk ZB, has described Christchurch as "eerily quiet."
“It’s a very surreal experience," the New Zealander told CNN.
"What many of us in Christchurch are struggling to comprehend is the graphic nature. We’ve had our experience of terrible natural disasters ... but it felt like the city was starting to feel alive again. This just takes us back. We all feel deflated. We feel like our city has gone through hell again. We feel defeated.
"It [the al Noor mosque] always played an important role in the community. Every year it opens its doors to the people of Christchurch. The people are lovely, generous people. It’s not a closed-off environment. It’s a very friendly mosque, the same with the mosque in Linwood. It’s an absolute tragedy for everybody.
"It’s not usual for us to have guns. We’re a strong hunting country but that’s as far as it goes. The right to hold arms is not in our nature," Lynch said.
Anna Coren, CNN's International Correspondent, has described the shootings as "unprecedented."
"New Zealand is a peaceful country," she said. "It does not see acts of violence, extremism on its streets. This is a country in shock and shaken to its core."
Muslim community groups worldwide are reacting to the tragedy in Christchurch, sending condolences to those affected and warning of the risk of further attacks at places of worship.
"This is indeed a very sad day for all," said I.H. Kauser, National President of the Ahmadiyya Muslim Community Australia, in a statement. "It is crucial that at this time we all remain united against hatred, division and bigotry."
American group Muslim Advocates said: "We are devastated. Today is a tragedy not just for Muslims, but for all people of faith and goodwill." The group urged Muslims in the United States to stay "vigilant and strong" as they attend mosques on Friday.
"This heinous attack is not an anomaly or a surprise ... the American Muslim community has faced deadly attacks in recent years, but rarely have we witnessed such brutal carnage as today's tragedy in New Zealand," its statement added.
"This is the most deadly Islamophobic terrorist attack we have experienced in recent times," said Harun Khan, Secretary General of the Muslim Council of Britain. "As the rest of us prepare to undertake our own Friday prayers today, we do so with the anxiety as to whether our mosques and communities are safe in the face of unabated Islamophobia and hostility against Muslims."
Khan called on fellow Muslims to "resist the temptation to roll up the banners in fear," and urged governments to step up efforts to ensure that mosques are protected.
"Police are currently in attendance at a property of Somerville Street, Dunedin," the New Zealand force said on Twitter. Dunedin is a city near the southern tip of New Zealand, around 225 miles (362 km) from Christchurch.
"This is a location of interest in relation to the serious firearms incident in Christchurch today," the police tweeted. "Evacuations of properties in the immediate area have taken place as a precaution." Cordons are in place around the area.
New Zealand Police have issued an update on the shooting, confirming that a 28-year-old man has been charged with murder and will appear in the Christchurch District Court on Saturday morning. Two others remain in custody.
"There are community events planned across the country this weekend and there will be a visible Police presence at these events for safety and reassurance," the force also said.