TORONTO - Thousands of people in downtown Toronto were left without power over the lunch hour Wednesday during an outage believed to have been caused by an underground cable.

In all, an estimated 20,000 hydro customers saw their power go down around 11:35 a.m. ET — an event that flooded the Internet with tweets about people trapped in elevators.

Still, the city and its major institutions took the blackout in stride. A major hospital switched to emergency power, subway trains and streetcars were unaffected, and the headquarters of the major banks managed to keep their lights on.

One Tim Hortons continued selling its remaining donuts to customers after its coffee ran dry.

With the G20 summit of world leaders coming to the city at month's end, at least one business saw it as an opportunity to put its summit backup plan to work.

"We've had to shut down our office and send everybody home," said Jeff Sciarra, who works at a technology company.

"But we have our G20 plan in place, and we're trying to enable that right now," said Sciarra, who added most people would be working from home.

The major disruptions expected from G20 security measures, and the threat of protests, reportedly has many downtown businesses making contingency plans, including having staff work from home during the event.

Samantha Hatton, who works for a web development company on Adelaide Street, said they too were being sent home early — but working from home wasn't part of the plan.

"I'm going to go to the mall and make the best of it," she said.

The blacked-out area was bordered by Bay Street in the west, Jarvis in the east, Shuter in the north and Lakeshore Boulevard in the south.

While lights flickered back on shortly after 1 p.m. for most of the area, it took another 20 minutes or so to bring three remaining buildings back up.

Toronto Hydro spokeswoman Tanya Bruckmueller said it was believed an underground cable fault was to blame.

The source of the problem was still unknown late Wednesday afternoon, Bruckmueller said.

The headquarters of the major banks were apparently not affected and the Toronto Stock Exchange was not hit by the outage.

St. Michael's Hospital was affected but operated on emergency power, a spokeswoman said.

Firefighters were called to several buildings to help people trapped in elevators.

People were also using Twitter to say they were stuck in elevators. One person tweeted there was someone trapped in an elevator at the CFL head office on Wellington Street, just east of Yonge.

Numerous traffic lights were knocked out in the affected area, causing traffic snarl-ups on streets already clogged with late morning traffic.

Toronto police spokeswoman Isabelle Cotton said officers were sent to those locations to keep traffic moving.