2 mosques were attacked in Christchurch, the gunmen stormed the Linwood Mosque and the Al Noor Mosque and shot down men who were praying for their Friday prayers.
On Saturday, outside one of the two mosques, 32-year-old Ash Mohammed pushed through police barricades in hopes of finding out what happened to his father and two brothers, whose cellphones rang unanswered.
Ardern said they were people she would describe as having "extremist views that have absolutely no place in New Zealand, or in fact, have no place in the world". They gave no details about those taken into custody except to say that none had been on any watch list.
Two explosive devices that were attached to the suspects' vehicles were also found at the scene. One has been defused and the second is in the process of being disabled.
Police have urged the public not to share the "extremely distressing" material online.
He told residents in Christchurch: "Close your doors until you hear from us again".
"It is clear that this can now only be described as a terrorist attack", Ardern said.
He added: " Out of respect and in condolence for all those killed in the terrorist attack in New Zealand, I have asked for flags to be flown at half-mast".
Police said they have mobilized every national police resource to respond to the incident.
Australian Prime Minister Scott Morrison has confirmed that one of the apprehended is an Australian-born citizen, while UK Prime Minister Theresa May has expressed her condolences in a statement.
Ardern confirmed that one of the attackers was Australian.
One man who said he was at the Al Noor mosque told media the gunman was white, blond and wearing a helmet and a bulletproof vest. The manifesto was not signed.
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Before Friday, New Zealand's worst mass shooting was in 1990 when a gun-mad loner killed 13 men, women and children in a 24-hour rampage in the tiny seaside village of Aramoana.
"New Zealand Police alerted us to a video on Facebook shortly after the livestream commenced and we removed both the shooter's Facebook account and the video". Reuters was unable to confirm the authenticity of the footage.
In the video, the killer spends more than two minutes inside the mosque spraying terrified worshippers with gunfire.
"It's a big mosque and there were more than 200 people inside".
He said: "An attack on a place of worship is an attack against all faiths, I'm confident that in the days ahead you will see all communities come together". One person was later released.
He said that he had a friend in another mosque in the area had told him a gunman had opened fire there as well and five people were dead.
Tarrant, who police say carried out at least one of the shootings, posted a jumbled, 74-page manifesto on social media in which he identified himself by name and said he was a 28-year-old Australian and white supremacist who was out to avenge attacks in Europe perpetrated by Muslims.
One man, with blood still on his shirt, said in a television interview that he hid from the gunman under a bench and prayed that he would run out of bullets.
Christchurch was put on lockdown with all schools and council buildings shut and roads closed across the city.
Christchurch is a coastal city of 404,500 residents. It has an agricultural economy.
The gunman then walks back into the mosque, where there are at least two dozen people lying on the ground.
Just 1% of New Zealand's population of nearly five million are Muslim, according to government statistics, less than 50,000 people in 2013.
"Just heard news of the devastating reports from New Zealand Christchurch".