Transit riders across Toronto were shocked to discover locked subway stations and out-of-service buses starting at midnight Friday after unionized Toronto Transit Commission workers walked off the job in an unexpected strike.

A news release from the Amalgamated Transit Union's Local 113 went out just over an hour before the strike began announcing 65 per cent of its members had voted against ratifying a tentative contract agreement reached last weekend.

"We will not expose our members to the dangers of assaults from angry and irrational members of the public." —Bob Kinnear, TTC union leader

Mayor David Miller blasted the union for failing to provide 48 hours' notice of a strike. "This is unacceptable and it's also irresponsible," he told a news conference.

Miller said Ontario Premier Dalton McGuinty has agreed to consider introducing back-to-work legislation.

"I informed the mayor that we will be proceeding with back-to-work legislation at the first available opportunity," McGuinty said in a release early Saturday.

The strike vote came as a surprise for many Torontonians who thought they were in the clear after the union, representing about 8,900 operating and maintenance employees, reached a last-minute deal last Sunday in time to avert a strike the next day. The deal had included a three per cent wage increase in each year.

At the time, commuters were given 48 hours' notice.

But Bob Kinnear, the head of the Amalgamated Transit Union, defended the suddenness of the walkout, saying it was to protect the safety of TTC workers.

Union wanted to avoid threats and abuse

He said in a news release that TTC workers were the target of threats and abuse from passengers last weekend after the union gave its original two days' strike notice.

"We have assessed the situation and decided that we will not expose our members to the dangers of assaults from angry and irrational members of the public," Kinnear said.

"We have a legal responsibility to protect the safety of our members and so does the TTC."

But many commuters were caught unaware late Friday night and were frustrated by the lack of notice this time.

Angela Dugas, 22, who works at the University of Toronto library, said her supervisor rushed over around 11 p.m. ET to tell her about the strike and allowed her to leave early.

"I'm just keeping my fingers crossed for Monday," said Dugas, who relies entirely on the TTC because she can't hold a driver's licence for medical reasons.

She said she's conflicted because she knows people who work for the transit service, but called the lack of notice "ridiculous."

Other commuters were arriving at stations to find doors already locked. Amber Brown was dropped off by friends at Woodbine subway station only to discover the TTC was on strike. "That's crazy," she said.

Labour ministry wants both sides to meet Saturday

"I don't think they should be allowed to go on strike," she said, adding that it will leave the city paralyzed.

About 750,000 riders use the TTC every workday.

The Ministry of Labour has asked both parties to return to the bargaining table at 1 p.m. Saturday.

The union said it will meet Saturday morning to discuss its next steps and wouldn't speak to the media until after then.

The strike may leave commuters scrambling to find ways to get to several major events in the city on Saturday, including the Toronto Raptors home game against Orlando Magic in the afternoon and the Green Living Show at Exhibition Place.