New Zealand's PM Jacinda Ardern says the three men and one woman in custody were not on any active watchlist and police are currently questioning them.
"Our thoughts and our prayers are with those who have been impacted today," says the prime minister.
"Christchurch was the home of these victims. For many this may not have been the place they were born. For many, New Zealand was their choice. A place they actively came to and commited themselves to.
"It was a place that many came to for its safety - a place they were free to practise their culture and religion."
New Zealand's PM Jacinda Ardern says 40 people have been killed.
The attackers' views "have no place in New Zealand and in fact no place in this world", says Ms Ardern.
She says there is no reason to believe there are other suspects at large "but we are not assuming that at this stage".
The national security level has been changed to "high".
PM Jacinda Ardern is speaking now. She says this can "only be described as a terrorist attack".
An account name in the name of the alleged attacker has been shut down on Twitter.
A video has emerged showing an arrest made earlier in the day in Christchurch, shortly after the mass shootings. New Zealand police have arrested four people so far.
A selection of images here from outside the Al Noor mosque, where an as yet unconfirmed number of people have died.
If you're just joining us, we're bringing you live coverage of a significant attack in Christchurch, New Zealand.
- Police say there have been multiple fatalities after attackers opened fire at two mosques. Four people are in custody.
- Large parts of Christchurch were under lockdown but that has now been lifted.
- A gunman is believed to have posted a manifesto online spouting hate against Muslims and minorities, and also livestreamed the attack on Facebook.
- Australia's PM has confirmed at least one of the attackers is an Australian-born citizen.
Facebook says it has taken down what is believed to be a live stream of the attack. But copies are still circulating online.
Police and organisations like Muslims in New Zealand are urging people to stop sharing the video, saying it will do more harm than good.
The shooting will likely put a close focus on the country's gun laws. It's a debate that's been going on for many years, in particular on how it's relatively easy to convert normal guns into military-style semi-automatic rifles.
According to a 2017 survey, New Zealand civilians own just over 1.2m firearms, that's 26 firearms per 100 people.
In Australia, the numbers are 3.5m overall and 14.5 firearms per 100 people while the data for England/Wales shows a total of 2.7m firearms which is only 4.6 for each 100 people.
In the United States, total number of firearms owned by civilians is almost 400m which is 120 firearms per 100 people.
Disturbing reports are emerging from people who witnessed the attacks. Several people talk about children being among the injured.
One witness has told stuff.co.nz he was praying in the Al Noor mosque on Deans Avenue when he heard shooting. He said he saw his wife lying dead on the footpath outside.
Another man said he saw children being shot: "There were bodies all over me."
An eyewitness told Radio New Zealand he heard shots fired and four people were lying on the ground, with "blood everywhere".
More from Scott Morrison:
"I can confirm that the individual who was taken into custody, I have been advised, is an Australian-born citizen. And obviously that element of the investigation Australian authorities are involved in and they will be proceeding with their investigation... involving all the relevant agencies."
One attacker is an Australian man, as the Australian PM Scott Morrison has just confirmed.
In a document posted online, a person claiming to be him, says he began planning the attack two years ago.
The 16,500-word document espouses far-right ideology and rails against "non-European" immigration in the West.
The man writes that he is not affiliated with any group, and does not intend to feel remorse for his actions.
Australia's PM Scott Morrison has called the attacker an "extremist rightwing violent terrorist".
This attack, he says, "reminds us of the evil that is ever present and would seek to strike out at any time".
Government data says Muslims make up around 1.1% of New Zealand's population. According to a 2013 census there were around 46,000 Muslims in New Zealand. The number has been rising, seeing an increase of 28% from 36,000 in 2006.
TVNZ reporter Anna Burns-Francis has told BBC World News that initial reports of an incident around the main hospital in Christchurch have not been confirmed by police.
The hospital is close to Al Noor mosque, she says, so those reports could have been the same incident.
This car, its windows shattered, was parked near the Al Noor mosque, where the first shooting happened.Copyright: AFP
It now appears clear that one of the gunmen shared a live stream of the attack on Facebook and posted content on Instagram.
Facebook has released the following statement about its response, saying it has taken down the content and is removing praise for the gunman.