The group were taken into custody after the gunman shot at worshippers as they gathered for Friday prayers, but the city remains locked down.

Police commissioner Mike Bush said it was still unclear if more people were involved.

Police say several people had been killed at two separate locations and have urged everyone across the country to avoid mosques.

Armed police were deployed around the Masjid Al Noor mosque where shooting broke out at 1.40pm (NZT) on Friday, with the city's schools and hospital locked down.

Reports later emerged of shots at another mosque in Linwood Avenue.

Unconfirmed media reports suggest dozens may be dead.

A witness, with blood splatters across his shirt, told AAP the shooter changed magazines seven times after opening fire as the crowd gathered to pray.

"When the shooting started people started rushing out, and the door was closed, and the guys came on them and started shooting them," he said, describing how he hid under a bench and pretended to stop breathing.

"He went to all the different [rooms] and he shot everyone."

Several hundred people were inside and witnesses reported seeing "blood everywhere".

One described seeing as many as 40 people injured.

"I heard a big sound of the gun. And a second one, I ran. Lots of people were sitting on the floor," a witness told TVNZ.

"The floor. There's lots of blood on the floor, you can see when you go in."

A 17-minute video that appeared to show a shooting, taken from a helmet camera, had been posted online with police urging the public not to share it.

"It is one of New Zealand's darkest days," Prime Minister Jacinda Ardern said.

"I would describe it as an unprecedented act of violence, an act that has absolutely no place in New Zealand."

Christchurch Mayor Lianne Dalziel urged residents to stay inside.

"Don't venture out. This is still an active situation," she said.

"I never could believe that something like this would ever happen in the city of Christchurch, but actually I would never believe that this would ever happen in New Zealand. And it looks as if something, the worst has happened, and we need to pull together and get through this situation."

New Zealand's top national security agencies held an emergency meeting in the afternoon.

Players from the Bangladesh cricket team, in Christchurch to play New Zealand in a Test match starting on Saturday, were in the mosque narrowly escaped the attack. They had been left shaken but uninjured, their coach told media.

The match since been cancelled.

Australian Associated Press