However, New Zealand police commissioner Mike Bush couldn't say at this point how many, only that it is significant. He addressed media a short while ago. 

During the news conference, Bush said the identities of all the victims were not known at this stage. He also said there were multiple improved explosive devices attached to vehicles as part of the attack. 

He said police were not assuming the situation was limited to Christchurch.  Four people have been arrested so far by police, three men and one woman.

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".

CNN and BBC