Prime Minister Jacinda Ardern, addressing the media about the Christchurch shooting, says there is "no place in New Zealand for such acts of extreme violence".
The PM said "it is clear this one of New Zealand's darkest days".
About 300 people were inside a Christchurch mosque for afternoon prayers when a gunman attacked them.
An eyewitness said there was "blood everywhere", adding that a man wearing a helmet and glasses and a military-style jacket opened fire inside the mosque with an automatic weapon.
Police say the "risk environment remains extremely high".
At the moment police have stated that they have one offender in custody, however, they have advised that there may be other offenders.
"Clearly what has happened here is an extraordinary and unprecedented act of violence," Ms Ardern said.
"Many of those who will have been directly affected by this shooting will be migrants, they will be refugees here. They have chosen to make New Zealand their home and it is their home. They are us.
"The person who has perpetuated this violence against us is not. They have no place in New Zealand.
"There is no place in New Zealand for such acts of extreme and unprecedented violence which it is clear this act was. "
She said her thoughts were also with those in Christchurch who were dealing with an unfolding situation.
"Please continue to listen out to information as it comes to light.
"Remain in lockdown, we are potentially dealing with an evolving situation as I said across multiple sites.
"Christchurch Hospital is dedicated to treating those who are arriving at the hospital as we speak."
She is scheduled to return to Wellington today.
People in central Christchurch have been urged to stay indoors and report any suspicious behaviour immediately to 111.