Police in New Zealand confirmed they were responding to reports of gunfire during Friday morning prayers.
At least 20 people are also understood to have been injured. Christchurch hospital reports it is treating 48 patients ranging from "young children to adults" for gunshot wounds.
The death toll is yet expected to rise.
Three men and a woman have been detained in connection with the attacks. New Zealand prime minister Jacinda Ardern has described the shootings as "terror attacks".
Controlled explosions have been carried out at the city’s Britomart station transport hub and cordons there are now being lifted. Train services were suspended following reports of gunfire.
The third test match between New Zealand and Bangladesh, which had been due to get under way on Saturday, has reportedly been cancelled after the Bangladesh team were caught up in the attacks.
The Foreign Office has updated its travel advice for New Zealand, stressing it is working with its local counterparts to seek further information and ensure any British travellers in the area are safe.
Said the FCO: "There has been a series of shootings in multiple locations in Christchurch. British nationals in the area are advised to remain vigilant and follow the advice of local authorities.
"The British High Commission in Wellington is in contact with the New Zealand authorities and urgently seeking further information."
Prime minister Theresa May said: "On behalf of the UK, my deepest condolences to the people of New Zealand after the horrifying terrorist attack in Christchurch. My thoughts are with all of those affected by this sickening act of violence."