Let’s not presume the danger is gone, police warn.

Of the four in custody, three are men and one a woman, says the police commisioner.

Police say the number of fatalities is significant, but did not give an exact number.

Police say four people are now in custody.

Australia's Prime Minster Scott Morrison has tweeted, saying he is "horrified" by the reports.

Earlier the police said one person had been detained but warned that more people might have been involved. So far there are also no definite numbers on fatalities nor injuries. There's another police statement coming up shortly.

Image source, Reuters

A manifesto alleged to have been written by the gunman is circulating online. It is full of anger against Muslims and migrants, talking in terms of an "invasion", and praise for white nationalists. It has echoes of the material posted by Anders Breivik, who carried out a deadly attack on a youth camp in Norway in 2011.

Police and anti-terror experts are urging people not to share the material further.

Authorities say multiple casualties were brought to Christchurch Hospital.

Image source, Reuters

Image source, Reuters

PM Jacinda Ardern is holding a crisis meeting of top security officials in Wellington, having flown back to the capital from a climate protest further north.

New Zealand's Stuff.co.nz website says the Officials Committee for Domestic and External Security Coordination, external (ODESC) is a group of chief executives of various emergency bodies which manages the response to national security. It only convenes in times of national crisis.

Police remind parents that all school across Christchurch remain in lockdown. That means parents can't pick up their children, who are being taken care of by the schools for now.

Christchurch Mayor Liannaer Dalziel has posted this message on Facebook.

She says the situation remains active and details are "sketchy". She has reassured parents that children under lockdown in school are safe. "We're not letting people in and we're not letting people out."

"It's really important that people feel absolutely calm and feel free to talk to friends and family about the feelings you will have. It's natural.

"It looks as if the worst has happened and we need to pull together and get through the situation."

She also apologises to deaf people, saying the lockdown means a sign language interpreter cannot be brought into the building.

Among those at the Al Noor mosque was the Bangladesh cricket team. All members are safely back at the hotel and in contact with the cricket board back home, the team said on Twitter.

Anna Burns-Francis, a reporter with TVNZ, has been speaking to BBC World News.

She says there is no gun culture in New Zealand - shops do not sell semi-automatic weapons as they do in the US, for example.

Firearms are generally used only in rural areas for hunting and "you wouldn't see anyone with a gun".

Messages of condolence are pouring in on the Facebook page of Muslims in New Zealand., external

Image source, facebook.com/muslims.newzealand

Image source, Facebook/MuslimsinNewZealand

Mohan Ibrahim, who was in the area of the Al Noor mosque, told the New Zealand Herald: "At first we thought it was an electric shock but then all these people started running.

"I have been calling my friends but there are many I haven't heard from. I am scared for my friends' lives."

A reminder that a huge march was taking place today. School children in Christchurch, as in cities across the world, were taking part in a rally demanding action on climate change. After the shooting, police told those in Christchurch to go home.

This was PM Jacinda Ardern speaking to children in Puke Ariki, north of Wellington, earlier.

There have been several clips circulating on social media purporting to show the first shooting - the New Zealand Police are now urging people to stop sharing it.

The Director of the UK's Islamophobia monitoring service Tell MAMA, Iman Atta, said he was appalled to hear about the New Zealand shooting.

"The killer appears to have put out a 'manifesto' based on white supremacist rhetoric which includes references to anti-Islamic comments. He mentions 'mass immigration' and 'an assault on our civilisation' and makes repeated references to his 'white identity'."

"Anti-Muslim hatred is fast becoming a global issue and a binding factor for extremist far right groups and individuals. It is a threat that needs to be taken seriously".

One witness described how the attack started just as everyone was getting settled for prayer in the Al Noor mosque.

“Then suddenly the shooting started. It started in the main room but I was in the side room so I didn’t see who was shooting. But I saw that some people were running out through the room I was in - some people had blood on their body and some were limping."

"At that moment I realized that it was really serious. I tried to get out and hid behind my car. The shooting went on for six minutes or more and I could hear crying and screaming."