A road sign is seen near the scene where the terror attacks occurred in Christchurch, New Zealand, March 16, 2019.
Two of the others remain in police custody, with a fourth person arrested deemed not to have been involved in the attacks.
The gunman sprayed innocent worshipers inside the Masjid Al Noor mosque in central Christchurch for more than two minutes before running back out to the street, where he takes aim at people down the sidewalk before returning to his auto for a different weapon.
Witness Len Peneha told The Associated Press he saw the gunman enter the mosque and then heard dozens of shots, followed by people running from the mosque in terror.
"They are us", she said.
"It is clear that this can now only be described as a terrorist attack", Ardern said, call this "one of New Zealand's darkest days".
All mosques in New Zealand had been asked to shut their doors, police said.
British security sources said there were no apparent United Kingdom links to the attack.
Four suspects have been taken into custody following the shooting and the gunman has been charged with murder.
In the video live-streamed by Tarrant, a man inside a mosque appears to say "Welcome brother" as a gunman approaches.
A manifesto posted online claiming responsibility for the attack is believed to have been written by suspected shooter, Brenton Harrison Tarrant, is still being probed by police in New Zealand. "They are us. The person who has perpetuated this violence against us is not". Police also defused explosive devices in a vehicle.
In full, Her Majesty said: "I have been deeply saddened by the appalling events in Christchurch today".
She added: "We have undoubtedly experienced an attack today that is unprecedented, unlike anything that we have experienced before".
There were just over 46,000 Muslims in New Zealand during the last census in 2013, a mere one per cent of the total population.
"And the one that we place the currency on right now is our compassion and support for the community of those directly affected by this tragedy".
"We will and must reject it". Dunedin is a city near the southern tip of New Zealand, around 225 miles from Christchurch. "Prince Philip and I send our condolences to the families and friends of those who have lost their lives".
'My heart goes out to the families of the murdered and all those impacted by this act of terror.
Police first received reports of the shootings at 1:40 p.m. Friday, local time, when mosques around the world are busiest as Muslims convene for Friday prayers.
Gov. Andrew Cuomo directed State Police to increase patrols near houses of worship, saying in a statement, "At a time of great division, we will stand up to hate in all its forms and do everything in our power to protect the safety and well-being of all New Yorkers".
"It's very disturbing, it shouldn't be in the public domain", a spokesman said.
One man, with blood still on his shirt, said in a television interview that he hid from a gunman under a bench and prayed that he would run out of bullets. "This has been done deliberately to also demonise legitimate Muslim political struggles", he tweeted.