Wellington - New Zealand police say extremely distressing footage of the mosque shooting in Christchurch has been published online.
"We would strongly urge that the link not be shared. We are working to have any footage removed," the statement said.
An extreme right-wing manifesto has also been released under the same name on other social media sites.
In it, the person claims to be a 28-year-old Australian man from a working-class family.
There were a "number of fatalities" on Friday following a shooting at a mosque in Christchurch and four people are in custody, New Zealand police confirmed.
"Police is responding to a very serious and tragic incident involving an active shooter in central Christchurch," Police Commissioner Mike Bush said.
"This is an evolving incident and we are working to confirm the facts, however, we can confirm there have been a number of fatalities.
"Police is currently at a number of scenes," he said. "We ask all mosques nationally to shut their doors, and advise that people refrain from visiting these premises until further notice."
Prime Minister Jacinda Ardern said it was one of New Zealand's "darkest days."
"This is an extraordinary and unprecedented act of violence," she told reporters.
She said many of those in the mosque were likely to be migrants. "New Zealand is their home. They should be safe here," she added.
The prime minister was speaking from the city of New Plymouth and said she would fly back to Wellington immediately to hold emergency meetings with her ministers.
One of the people who was at afternoon prayers at the mosque on Deans Avenue in central Christchurch told Radio New Zealand a man came in and opened fire 10 minutes into their prayers.
"The guy just started firing shots, we all took for cover. We didn't hear no more shots and we got up and obviously some people from the mosque ran out, and they came back in a pile of blood, some had been shot and then about five minutes after that the police turned up and they escorted us outside," he said.
Several hundred people were at the mosque for Friday prayers, witnesses told local media.
Police recommended that residents across Christchurch remain off the streets and indoors until further notice while all schools were in lockdown until further notice.
The city's hospital was also in total lockdown. "All appointments have been cancelled this afternoon, and no staff or patients are to enter or leave the building," the hospital said in a statement.
There were also unconfirmed reports of shootings at other locations as well as a bomb found in a car crashed on a central street near the attack.
The former president of the Muslim Association of Canterbury, Mohammed Jama, who was at the mosque, told local news website Stuff he saw about four people injured and two people lying on the ground.
He did not know if they were alive or dead.
Witness Ahmad Al-Mahmoud told Stuff the shooter was white-skinned, blond, quite short and wearing a helmet and a bulletproof vest.
He reportedly had an automatic gun and emptied at least two magazines.
Bangladeshi cricketer Tamim Iqbal said that the entire national team had been at the mosque.
"Entire team got saved from active shooters!!! Frightening experience and please keep us in your prayers," he wrote on Twitter.
New Zealand's last mass shooting was in 1990 when David Gray shot 13 people in the Aramoana massacre.