Surrounding schools are in lockdown, with armed police deployed at the scene near Hagley Park.
Christchurch Hospital and the nearby outpatients building are in lockdown.
Police Commissioner Mike Bush said "a serious and evolving situation was occurring in Christchurch with an active shooter".
He said the gunman was white and was wearing a helmet with some kind of device on top, giving him a military-type appearance.
New Zealand Prime Minister Jacinta Arden said it was "one of New Zealand's darkest days". "I would describe it as an unprecedented act of violence". An act that has absolutely no place in New Zealand. But she says it is clear so far "this is a significant event". And a second one, I ran.
Adern confirmed that one person had been taken under custody.
According to a stuff.co.nz report, a gunman reportedly opened fire at the mosque around 11.40am (AEDT).
The man, who did not give his name said he saw four people on the floor but did not see the shooter.
"There were bodies all over me", he said.
Gunfire was heard coming from the direction of the mosque and several people were seen on the ground outside the building, One News reported.
In response to "a serious ongoing firearms incident", police said all schools have been placed in lock-down.
New Zealand Police have stormed the area and are urging anyone in the area to stay indoors.
Ibrahim said people began running out of the mosque and he "had to jump out of the wall" to escape.
Members of the Bangladesh cricket team after the attack.
Tamim Iqbal Khan, a member of the team, tweeted that the "entire team got saved from active shooters".
Mario Villavarayen, strength and conditioning coach of the Bangladesh cricket team said the players were shaken up, but fine.
"I don't they're in a mental state to play cricket at all". Liton Das and Nayeem Hasan were also back in the hotel and have been contacted, and told to stay back at the hotel.
"Just escaped active shooters!"
'Frightening experience and please keep us in your prayers, ' Tamim Iqbal tweeted.