A six-alarm fire burns in a highrise apartment building at 200 Wellesley St. E. in downtown Toronto. (CBC)

Hundreds of Toronto residents of a downtown highrise apartment were forced from their homes following a six-alarm fire that took firefighters hours to extinguish.

At least 12 people were taken to hospital, three in serious condition, because of  the blaze, that started on the 24th floor of the 30-storey building at 200 Wellesley St. E. near Bleecker Street.

Toronto Fire Service officials said the extreme heat from the fire, which began at about 5 p.m., caused spot fires in other areas. It wasn't until late into the evening that the fire was brought under control.

Shelter was being arranged for the tenants who were evacuated from the building.

Fire crews were still arriving 90 minutes after the fire began. At least 120 firefighters were needed to douse the flames.

The highrise is part of Toronto Community Housing, the largest social housing provider in Canada.

The building is in the St. James Town neighbourhood. According to the non-profit think tank the Wellesley Institute, St. James Town is one of Canada's most densely populated neighbourhoods, with 18 highrises, most built in the late 1960s.