MILWAUKEE — Hundreds of Milwaukee County union bus drivers went on strike early Wednesday, leaving tens of thousands of commuters scrambling to make alternative plans to get to work and elsewhere.

The roughly 750 drivers walked off the job at 3 a.m. and began picketing at the Milwaukee County Transit System garages after contract negotiations between union leaders and transit officials broke down. The sides met with a federal mediator Tuesday in a last-ditch effort to avoid a strike.

Amalgamated Transit Union Local 998 leaders said the strike would last until Saturday. It is the first strike by MCTS drivers since 1978, when union members stayed off the job for 39 days.

Riders, including 20-year-old Tashanita McCall, of Milwaukee, were left without a low-cost transportation alternative.

"I'm used to taking the bus to school every morning," said McCall, who attends Bryant and Stratton College in downtown Milwaukee. She planned to ask neighbors or relatives for a ride to school.

At Planet Fitness in downtown Milwaukee, assistant manager Tess Elkins was worried how her employees would get to work.

"They don't have any other way to get here. They don't have rides," Elkins said.

Union President James Macon said drivers are getting support from the public, who waved or honked their horns at picketing drivers on a major Milwaukee street.

The system's 450 buses are parked in MCTS garages throughout the city and bus service is shut down, transit system spokesman Brendan Conway said.

Conway said the transit system is fielding "a lot of concerns and complaints. People have told us they don't know how they'll get to work. ... And we agree, we understand. We wish there were more that we could do."

The transit system offered concessions Tuesday, including a cap on the number of part-time drivers it would hire. But the union pushed for higher wages that would have cost $8 million more a year, according to transit officials.

Milwaukee County Executive Chris Abele accused the union of "punishing" people who rely on buses to get around. Abele said the county transit system made its best offer Tuesday.

"Anything more than this is unsustainable in our County budget and would be irresponsible to the taxpayers we serve," Abele said in a statement.

Local 998 vice president Rick Bassler said the union didn't want to strike but that transit system negotiators were "forcing things down our throat."

Transit system officials want to hire a limited number of part-time drivers to cut overtime hours for full-time union members. Union leaders said part-time drivers would have no health or pension benefits. Drivers are paid an average hourly wage of $23.78. The latest transit proposal would increase wages to $24 an hour in 2016 and $24.45 in 2017.

The transit system serves about 150,000 passengers a day, with an additional 20,000 or more daily during the Summerfest lakefront music festival, which ends Sunday.

A 2015 study commissioned by Milwaukee County estimated 5,000 people would eventually lose their jobs if they couldn't rely on the transit buses. The MCTS amounted to a $342 million a year benefit to the metropolitan area because of savings from taxi fares, travel time, parking, fewer accidents and not owning a vehicle.