Forty nine people were killed and 20 wounded in what the prime minister of New Zealand called a "terror attack" on two mosques in the city of Christchurch on March 15.
Speaking to reporters, Jacinda Arden said 30 people were killed at the Masjid Al Noor mosque and 10 were killed in a mosque in the suburb of Linwood.
"It is clear that this can only be described as a terrorist attack," Ardern said.
Ardern said New Zealand had been placed on its highest security threat level. She said four people in police custody held extremist views, but had not been on any police watchlists.
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 man burst into the mosque as worshippers were kneeling for prayers.
"He had a big gun ... he came and started shooting everyone in the mosque, everywhere," said the man, Ahmad Al-Mahmoud. He said he and others escaped by breaking through a glass door.
Radio New Zealand quoted a witness inside the mosque saying he heard shots fired and at least four people were lying on the ground and "there was blood everywhere".
Australian Prime Minister Scott Morrison confirmed that one of the detained suspects was an Australian.
A man who claimed responsibility for the shootings said in a manifesto that he was a 28-year-old white Australian who came to New Zealand only to plan and prepare for the attack.
Police said IEDs, improvised explosive devices, were found with a vehicle they stopped.
All mosques in New Zealand had been asked to shut their doors, police said.
The Bangladesh cricket team was arriving for Friday Prayers when the shooting occurred but all members were safe, a team coach told Reuters.
In earlier comments, Ardern said the events made for "one of New Zealand's darkest days."
“What has happened here is an extraordinary and unprecedented act of violence," she added.
With reporting by AP, Reuters, and dpa