Gunshots have been heard near the Al Noor mosque in Christchurch, New Zealand, local media report, as police warn of a "critical incident".
Witnesses told local media that several people appeared to be injured inside the building. Police have warned people to avoid the area.
All schools in Christchurch have been placed on lockdown, police commissioner Mike Bush said.
Eyewitnesses have reported running for their lives to escape a shooter.
Mohan Ibrahim 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."
The Al Noor mosque is located in central Christchurch along Dean Avenue, facing Hagley Park.
Witnesses have reported seeing bodies, but this has not been confirmed by police or officials.
There are reports that a second mosque has been evacuated.
Officers have reportedly also cleared Cathedral Square, where thousands of children were holding a rally for action on climate change.
Police Commissioner Mike Bush said: "A serious and evolving situation is occurring in Christchurch with an active shooter.
"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."