New Zealand is in mourning after 49 people were killed and more than 20 wounded at two mosques in Christchurch in a March 15 attack that Prime Minister Jacinda Ardern called one of the country's "darkest days."
Jacinda Arden said New Zealand had been placed on its highest security-threat level. She said four people in police custody, three men and one woman, held extremist views, but had not been on any police watchlists.
No names have been made public.
"It is clear that this can only be described as a terrorist attack," Ardern said.
Police Commissioner Mike Bush said 49 people had been killed at two mosques and one man in his late 20s charged with murder.
A gunman believed to be a white supremacist live-streamed the killing spree from a head-mounted camera.
The gunman, who said he was a 28-year-old Australian called Brenton Tarrant can be seen shooting at men, women, and children. He posted a 74-page manifesto online.
U.S. President Donald Trump tweeted, "We stand in solidarity with New Zealand" and pledged to give the country any assistance it needs.
Afghanistan's ambassador to Australia, New Zealand, and Fiji said on Twitter that one Afghan national had been killed and three others wounded in the attack.
Mirwais Waziri, an Afghan national wounded in the attack at the Masjid al-Noor mosque, described to RFE/RL his efforts to escape.
"The mosque had two emergency exits. I got to one of the exits. The firing continued. People were lying there and there was a crowd at the exit doors. I also laid down along with other men. A bullet passed by my head but left a small wound. Thank God that I survived but many others were martyred and injured," Waziri said.
Another man who said he was at the Masjid 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 as saying he heard shots fired and at least four people were lying on the ground and "there was blood everywhere."
WATCH: One eyewitness describes the moment an attacker started shooting at one of the mosques.
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 improvised explosive devices were found with a vehicle they stopped.
All mosques in New Zealand had been asked to shut their doors, police said. Flowers have been placed outside mosques across the country.
The Bangladeshi 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.
Turkish President Recep Tayyip Erdogan called on Western countries to rapidly take measures to curb rising racism against Islam and Muslims, saying new attacks like the one in New Zealand would otherwise be "inevitable."
Pakistani Prime Minister Imran Khan blamed what he called rising Islamophobia for the violence in New Zealand.
Writing on Twitter, Khan said that "terrorism does not have a religion."
New Zealand, with 5 million inhabitants, has relatively loose gun laws but few gun homicides, and is generally considered to be welcoming to migrants and refugees.