There are "multiple fatalities" after shootings at two mosques in the city of Christchurch, police in New Zealand have confirmed
Police commissioner Mike Bush confirmed that one person is in custody, but said it was unclear if others were involved.
Witnesses told local media they ran for their lives, and saw people bleeding on the ground outside the Al Noor mosque.
Authorities advised all mosques to shut down until further notice. All Christchurch schools are on lockdown.
In a statement to the nation, Prime Minister Jacinda Ardern said details were not yet clear, "but I can tell you now - this will be one of New Zealand's darkest days".
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 still have friends inside.
"I have been calling my friends but there are many I haven't heard from. I am scared for my friends lives."
It is not yet known how many shooters there were, but the Herald reports that one gunman is believed to be an Australian who has written a manifesto outlining his intentions. In it, he espouses far-right ideology and anti-immigrant ideology.
Unverified footage purportedly taken by the shooter has emerged, suggesting he filmed as he shot victims.
What happened at the mosques?
The sequence of events remains unclear and has mostly come via eyewitness reports to local media.
Witnesses at the Al Noor mosque, located in central Christchurch along Deans Avenue, facing Hagley Park, reported seeing people bleeding on the ground outside the building, but such details have not been confirmed by police or officials.
One unnamed survivor told TV New Zealand he saw a gunman shoot a man directly in the chest. He estimated that the shooting lasted for 20 minutes and that up to 60 people may have been injured.
The gunman reportedly targeted the men's prayer room in the mosque, then moved to the women's room.
"What I did was basically just waiting and praying, God please, let this guy run out of bullets," the witness said.
"He came to this side, he shot this side, he went to another room and went to the ladies' section and shot them. I just heard one of the ladies has died."
A second mosque in the suburb of Linwood has also been evacuated, and the police commissioner said "multiple fatalities" were recorded at two locations. There are far fewer details emerging from there, however.
How are the authorities responding?
A spokeswoman said Canterbury District Health Board (CDHB) has activated its mass casualty plan, according to New Zealand news site Stuff.nz.
The plan includes clearing emergency room space for casualties. The spokeswoman did not comment on how many patients were expected.
Police have cleared Cathedral Square, where thousands of children were holding a rally for action on climate change.
Police Commissioner Mike Bush said: ""Police are responding with its full capability to manage the situation, but the risk environment remains extremely high.
"Police recommend that residents across Christchurch remain off the streets and indoors until further notice. Christchurch schools will be locked down until further notice.
"Police thanks the public for their cooperation and will provide further updates to keep residents informed."
Australia's Prime Minister Scott Morrison tweeted: "I'm horrified by the reports I'm following of the serious shooting in Christchurch, New Zealand. The situation is still unfolding but our thoughts and prayers are with our Kiwi cousins."