All mosques in New Zealand had been asked to shut their doors, police said.
One more person died later at Christchurch Hospital.
After parking, he took two guns and walked a short distance to the mosque where he opened fire. Health authorites said 48 people were being treated for gunshot wounds, including young children. Gray also said she recalled that Tarrant's father Rodney died from some kind of asbestos-related illness when Tarrant was finishing high school.
Authorities have not specified who they detained, but said none had been on any watch list.
Police said improvised explosive devices were found with a vehicle they stopped. One has been defused and the second is in the process of being disabled.
"This goes to the seriousness of the situation", Bush said.
Police have urged people not to share the "distressing" footage.
Christchurch, a relatively small city in the south of New Zealand, hit global headlines in 2011 when it was struck by a deadly quake.More news: Pompeo: Israel Has Right to Defend Itself If Attacked
"It is clear that this can only be described as a terrorist attack".
The Bangladesh cricket team - which had been in Christchurch for a test match against New Zealand that was later cancelled - all escaped without injury.
This was to ensure all New Zealand's intelligence agencies were responding in the most "appropriate way" - including at the border. "As you can imagine, this is requiring every police and emergency resource we have available".
Australian prime minister Scott Morrison said one of the suspects in the "rightwing extremist attack" was an Australian-born citizen. The manifesto was not signed. He did not say whether police believed the same shooter was responsible for both attacks.
None of the four people arrested were on New Zealand's security and terrorism watchlist. "We are working to have any footage removed". "We're also removing any praise or support for the crime and the shooter or shooters as soon as we're aware", she said.
CNN has not been able to independently confirm any information about any of the attackers and the alleged video at this stage.
- Officers first received reports of shots fired in central Christchurch at about 1.40pm local time (12.40am GMT).
In the video, the killer spends more than two minutes inside the mosque spraying terrified worshippers with gunfire.More news: Wales coach Warren Gatland: It was a fantastic performance
He describes himself as "a private and mostly introverted person" and admits he is racist, adding that he is an "Eco-fascist by nature". The gunmen came from the backside. "I saw lots of broken glass and bricks on the backside of the mosque", he said.
One eye witness said that the gunman entered from the main entrance of the building and began shooting.
"I could not contact two of my friends who are in the mosque as well", he said. He said one was slightly injured.
"I was thinking that he must run out bullets you know, so what I did was basically waiting and praying to God, oh God please let this guy run out of bullets", he said.
Christchurch city council offered a helpline for parents looking for kids attending a mass climate change rally nearby.
Christchurch is a coastal city of 404,500 residents.
Home to almost 400,000 people, Christchurch is the largest city on New Zealand's South Island. It has an agricultural economy.
Brenton Tarrant posted a 73-page manifesto on twitter before he started to kill people, he described them as a terrorist attack. Innocent people being murdered as they worship is horrific and heartbreaking.More news: Sandy Hook families can sue gunmaker, says court