"It is clear that this can now only be described as a terrorist attack," said Prime Minister Jacinda Ardern, describing it as "one of New Zealand's darkest days."
One of the gunmen -– believed to be an Australian extremist -- livestreamed the deadly assault, stoking outrage and fear that others may be targeted in copy-cat attacks.
Here is a summary of the main international reactions so far.
Muslims face 'mass killing'
"With this attack, hostility towards Islam that the world has been has been idly watching and even encouraging for some time, has gone beyond the boundaries of individual harassment to reach the level of mass killing," said Turkish President Recep Tayyip Erdogan.
"If measures are not taken right away, news of other disasters will follow this one... I am calling on the world, in particular the West, to take quick measures," he said.
Norwegian Prime Minister Erna Solberg urged the international community to combat all forms of extremism after the Christchurch attacks, which revived painful memories of the 2011 mass killings in Norway by rightwing extremist Anders Behring Breivik.
"It's obviously very sad. It recalls painful memories of our own experience with July 22, the most difficult moment in the post-war period in Norway."
'Do the necessary'
Malaysian Prime Minister Mahathir Mohamad said he hoped New Zealand "will arrest these terrorists and do the necessary under the law of the country."
Indonesian President Joko Widoyo, head of the world's largest Muslim country, said "we strongly condemn these kind of violent acts".
"Harrowing news from New Zealand overnight" said EU Council president Donald Tusk.
"The brutal attack... will never diminish the tolerance and decency that New Zealand is famous for."
'Sickening act of violence'
British Prime Minister Theresa May offered deepest condolences "after the horrifying terrorist attack in Christchurch. My thoughts are with all of those affected by this sickening act of violence."
London's police service said it was "stepping up reassurance patrols around mosques and increasing engagement with communities of all faith, giving advice on how people and places can protect themselves."
In Australia, police in New South Wales said there were increasing patrols around mosques as a precaution.
"There is no ongoing or specific threat to any mosque or place of worship," police said.
Cruel, cynical attack
"An attack against peaceful people gathering for prayer is shocking in its cruelty and cynicism," Russian President Vladimir Putin said.
"I hope that those involved will be severely punished," he said in a message to Arden.
German Chancellor Angela Merkel said she mourned "with New Zealanders for their fellow citizens who were attacked and murdered out of racist hatred while peacefully praying in their mosques. We stand together against such acts of terrorism."
French President Emmanuel Macron echoed Merkel's message, condemning an "odious attack" and saying France "stands against any form of extremism".
NATO chief Jens Stoltenberg said the US-led alliance "stands with our friend and partner New Zealand in defence of our open societies and shared values".
'Fanatics want to destroy society'
Spanish Premier Pedro Sanchez said his thoughts were with the victims, families and government of New Zealand after attacks by "fanatics and extremists who want to destroy our societies".