A power outage and a failure of a backup power supply caused Monday's crippling subway shutdown, Toronto Transit Commission's CEO Andy Byford said on Monday afternoon.

A power surge hit the TTC's Hillcrest complex, which houses the main control centre, on Sunday night, forcing the system to rely on its uninterruptible power supply.

LIVE BLOG | TTC subway service shut down

A circuit board failure then caused the backup system's battery to drain completely around 5:30 a.m. ET, at which point the TTC's communications systems went down, triggering the subway shutdown which lasted until 7:35 a.m. and left thousands of commuters stranded during the rainy rush hour.

It is deeply troubling that our system can experience such a major technical failure. - Coun. Josh Colle, TTC chair

Byford said there was no way the trains could run without the communications system, likening the situation to trying to land planes without air traffic control radios.

The outage also stopped the TTC from alerting passengers about the major issues through its online and telephone systems.

"I absolutely understand customers' frustration," Byford told reporters at an afternoon news conference.

"Our reputation took a big hit this morning."

Shuttle buses, which are often the only alternative for transit riders during subway closures, didn't run during the service suspension, because it wasn't possible to replace the entire subway's capacity with buses, Byford said.

City officials were quick to sound the alarm about the shutdown.

"Passengers' safety is paramount and shutting down the subway system in lieu of the ability to communicate with subway trains was the responsible thing to do," said TTC chair and Toronto Coun. Josh Colle in a statement.

"It is deeply troubling that our system can experience such a major technical failure," he said.

Line 3, the Scarborough Rapid Transit (SRT) line — which runs partly above ground in the east end of the city — was the first to begin operating after the shutdown. Line 1 Yonge-University, Line 2 Bloor-Danforth and Line 3 Sheppard followed shortly after.

Uber 'surge pricing' kicks in

During the peak of the service outage, Toronto taxi companies sent cars to transit stations throughout the city, however many commuters told CBC News that they were still unable to find an empty car.

The ride-sharing service Uber was charging fares several times the average of a normal morning.

Under the so-called "surge" or dynamic pricing system, the ride-hailing service uses an algorithm to lure more drivers to areas where demand is particularly high by increasing the rates in those areas.

The practice has incited controversy among some users who have called it "price gouging."

Monday morning, Uber was charging up to four times the usual rate in some parts of the city.

But Uber spokeswoman Susie Heath says that as soon as the company became aware of the transit shutdown, it capped its dynamic pricing at three times the normal rate.

'Every day it's something'

Speaking with reporters at city hall, Toronto Mayor John Tory said TTC officials have assured him that a "thorough investigation" is underway into how both the primary and backup communication system could fail simultaneously.

"These things are happening more often than they should in a sophisticated transit system in a big city," he said.

Some commuters said they felt the TTC's failure to communicate the extent of the problem adequately to passengers made the situation worse.

"I'm trying to give the TTC benefit of the doubt, but every day it's something," said Darlene, a passenger who spoke with CBC News outside of Kipling Station in the city's west end and only gave her first name.

Many people were left stranded for more than an hour during the shutdown. Shuttle buses were not running during the service outage. (Sara-Christine Gemson/CBC) "There is always some kind of issue, and they don't give you a refund," she said. "It's ridiculous. Another not-too-happy day on the TTC."

Throughout the city, packed subway stations were evacuated as crews worked to restore service. Passengers tried to flag down taxis or work out an alternative route on other public transit services.

"It's sickening how much money we pay every month and you can't rely on decent subway service," said rider Mary Kate Markey.

"They keep hiking the fares but the service doesn't get any better. It's pretty annoying at this point."

Both the University of Toronto and Ryerson University were forced to delay morning graduation ceremonies by at least an hour to accommodate students who were late because of the shutdown.

Pritesh Soni, an international student at Centennial College in Scarborough, was trying to get to campus to write an exam this morning.

"I had to call my brother, who is at work, to come and pick me up and take me to school," he said. "I have an exam. I have absolutely no choice but to get there."