Police said multiple fatalities had occurred at two mosques, but it was unclear how many attackers were involved.
UPDATE: 12:18PM: Police are warning locals about a possible bomb found in a crashed vehicle near the shooting.
Speaking to media after police had confirmed that one man was in custody following mass shootings at two mosques in the South Island city, Ardern said those responsible "have no place in New Zealand".
The Bangledeshi cricket team was in the area at the time of the shooting.
"The multiple fatalities are, as far as we know, at two locations", he said.
Armed police have been deployed after receiving reports of shots fired in the city center at 1:40 p.m. Friday local time.
Radio Tarana's Pawan Prasad says Christchurch is in lockdown at this stage.
He said the man shot several elderly people sitting on chairs during prayer. It was an unprecedented act of violence.
Bangladesh Cricket Board spokesman Jalal Yunus said most of the team were bussed to the mosque and were about to go inside when the incident happened.
Commissioner Mike Bush said it was a "very serious and grave situation" and warned people not to leave their homes.
"At first we thought it was an electric shock but then all these people started running". "Some were covered in blood".
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Once he was satisfied everyone was dead, he ran outside and shot another person he saw on the mosque's front lawn.
"Police are now responding to reports of shots fired in central Christchurch", the police said in a statement.
Central city buildings, including the Civic Offices and Central Library, were also locked down. Nearby climate change protests were evacuated and council staff have been looking after children separated from their relatives in the chaos.
Ten emergency service vehicles are at the scene and police said it is an "evolving situation".
Bangladeshi cricket player Tamim Iqbal said that the entire national team had been at the mosque.
"They are safe. But they are mentally shocked".
Mr. Tarrant decries "mass immigration" as "white genocide" and says why he commits the attack.
"It is clear that this is one of New Zealand's darkest days".
In a lenghty manifesto published online the supposed shooter outlined who he was and why he carried out the massacre at the Christchurch mosque.
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