Facebook said it had removed the video following a police alert and blocked the alleged shooter's Facebook and Instagram accounts.
The fallout from the attack featured all the astringent elements that have become hallmarks of such modern acts of nihilistic violence: a discussion of the negative externalities of a globalized world, a left-wing quick to blame firearm proliferation, a right-wing eager to highlight spiritual disrepair, and social media behemoths seeking but struggling to contain internet hysteria. "We are working to have any footage removed".
YouTube's handling of the footage drew criticism from British Labour Party Deputy Leader Tom Watson, who argued that if the site can't halt the dissemination of the videos, it should suspend new uploads across the board.
Social networks have been caught flat-footed in many cases by videos showing violent acts including suicides and assassinations. "We will continue working directly with New Zealand Police as their response and investigation continues". She said the attacks were shown live on Facebook for 17 minutes before being stopped.
A suspected gunman broadcast live footage on Facebook of the attack on one mosque in the city of Christchurch, mirroring the carnage played out in video games, after publishing a "manifesto" in which he denounced immigrants.
"The stream is not analysed, stored or processed by LIVE4 in any way, we have no ability (even if we wanted to) to look at the live streams as they are happening or after it's completed", he said in written comments to Reuters.
"The responsibility for content of the stream lies completely and exclusively on the person who initiated the stream".
Prime Minister Jacinda Ardern said the events in Christchurch represented "an extraordinary and unprecedented act of violence", and that numerous victims could be migrants or refugees, according to The Associated Press. "That's unacceptable, it should have never happened, and it should have been taken down a lot more swiftly".
A spokesman for New Zealand's interior ministry said the video is likely to be classified as objectionable content under local law, and could be illegal to share. "The content of the video is disturbing and will be harmful to people to see", he said.
YouTube said: "Please know we are working vigilantly to remove any violent footage".
But private online communities dedicated to violent content were still looking for ways to share copies of the video. "Groups have deliberately spread it and those accounts should be closed down".
Google has been condemned for not immediately taking down graphic footage of the terrorist attack on two mosques in New Zealand.
"There's no excuse for the content from that livestream to be still circulating on social media now", said Lucinda Creighton, a former government minister in Ireland and an advisor to the Counter Extremism Project, which campaigns to remove violent internet content. Children's screams could be heard in the distance as he strode to his auto to get another rifle, then returned to the mosque, where at least two dozen people could be seen lying in pools of blood.