Canadian authorities today ordered the mass evacuation of more than 200,000 people from this industrial town near Toronto after a freight train derailed and tanker cars caught fire, raising the possibility of an explosion of poison chlorine gas.

Authorities said there were no major injuries as a result of the fire that started when the train derailed late last night and seven tanker cars containing propane and other gases exploded.

But nausea, headaches and watery eyes were common complaints from firefighters and others at the site of the blaze.

"There's no doubt in my mind that this is the biggest evacuation in the history of North America," said police superintendent Karl Barnhart.

Authorities said they feared a 90-ton tanker filled with chlorine might explode as the cars around it continued to burn. Chlorine gas has been seeping from the tanker since the accident and police said winds of increasing strength could carry the gaseous smoke farther afield.

Provincial authorities stressed that the advice to evaculate all 240,000 residents of Mississauga, was "entirely precautionary." The evacuation covered about 40 square miles.

There were unconfirmed reports that the train left the tracks after colliding with a tractor-trailer and Ontario Solicitor General Roy McMurtry said "you can be sure that there will be the most thorough investigation made of the whole incident."

About 200 troops were called into the area to guard against looting.

Among the evacuatees were some 400 people at Mississauga General Hospital and several hundred people at an extended care nursing home.

Several thousand people in the metropolitan Toronto borough of Etobicoke also were evacuated.

Some evacuees went to schools and shopping centers in nearby Streetsville, where emergency operations were set up.

At first, city officials said they expected to allow residents of the Mississauga area -- about 10 miles west of Toronto -- to return to their homes Monday, but when the mass evecuation order was given tonight, they said that was doubtful.

A civil engineer with the Environment Ministry said there was no immediate danger from the release of chlorine gas because the rate of emission was so slow.