Thousands of Toronto residents were without power Tuesday after torrential rains caused widespread outages in Canada's largest city, prompting the mayor to call for energy conservation.

The utility company Toronto Hydro said 300,000 people were left without power overnight but that as of morning all but 20,000 had it restored.

Mayor Rob Ford insisted, however, that it was "crucial" for residents to cut back on their electricity use to relieve strain on the system.

"We're hanging on by a thread right now," he told a press conference. "Please turn off all non-essential electronics and lighting."

Basements and highway underpasses were flooded, leaving some people trapped in cars with water up to their windows.

The city's subway and streetcars lost power, several trains were trapped in tunnels and tracks were inundated, officials said.

Public broadcaster CBC said about 1,400 people were caught by the flooding aboard one train, prompting an hours-long rescue operation by police and firefighters using small, inflatable boats.

The record rainfall of 110 millimeters per hour was equivalent to the amount of rain that usually falls on the city in a month.

At noon Tuesday (1600 GMT) several roads remained closed due to flooding or fallen trees.

Power was expected to be fully restored by Wednesday and the subway was back up except for a small leg still under water, officials said.