TEMPE, Ariz. — Tens of thousands of bus riders in Phoenix's sprawling eastern suburbs were left without rides to work Thursday after drivers went on strike in a dispute with the company that just took over operations for the area's regional transit system.

Buses serving 40 routes in Chandler, Gilbert, Mesa and Tempe were halted, as well as express routes from those cities to Scottsdale and Phoenix, leaving some 57,000 daily riders in the lurch as temperatures approached 110 degrees. The area served by the striking drivers covers hundreds of square miles.

Riders like landscaper Jesse Garcia, 23, who was sitting at a Tempe light rail station and getting ready to walk several miles home to Mesa, was upset but supportive of the drivers.

"It's understandable in a way, but I kind of wish they didn't cut all of them off, and left at least some of the buses running," he said.

Drivers on the picket line in front of Tempe bus yard said their biggest issue with First Transit, which took over operations for part of the Valley Metro system July 1, was job security.

The contract between the regional transit authority and First Transit would override First Transit's proposed deal with the union, because of a clause within the labor contract, according to union members.

"It doesn't protect our members, it doesn't protect any of the operators," said James Washington, a driver for 23 years who serves on the board of Amalgamated Transit Union's Local 1433. "It gives management the right to run rampant and do whatever they like and their client basically dictates what they need to do to us."

A spokeswoman for First Transit, Jen Biddinger, said the company offered a good deal to the union members and is hoping a federal mediator can bring the two sides back to the bargaining table.

"We presented several offers to the union, which included higher wages and lower health care costs," Biddinger said.

She also said she thought a deal had been reached on the so-called management rights issue.

ATU Local 1433 president Bob Bean said no deal has been reached, although a new offer on the "management rights" clause was sent to the union Thursday.

He said the original contract language would have allowed the company to convert all drivers to part-timers, cutting benefits and wages. The low bid by First Transit — which cut $77 million over 10 years from the previous contract — telegraphed the company's intentions, according to Bean.

Bean later said that the strike was continuing, but added that "we are optimistic that an agreement will be reached."

Union members rejected the latest offer by a 95 percent margin Wednesday. The strike by about 400 drivers doesn't affect buses in Phoenix and western suburbs, or the region's single light rail line, which runs from north-central Phoenix to Mesa.

But once riders like 18-year-old Audrayona Adair got off the train, they found themselves stranded. While surprised to find out about the strike as she made her way home to Chandler, Adair was generally supportive of the drivers. But, she added, "now we're kind of just stuck, we can't get anywhere."

The Valley Metropolitan Regional Transit Authority said it was not involved in negotiations between First Transit and the union. First Transit won a three-year contract from the authority with options for seven additional years in January.

The contract involved combining operations in Mesa and Tempe, and involved savings of $36 million over the previous contract by operating both parts of the system together and streamlining operations, Valley Metro spokeswoman Susan Tierney said.

Striking workers said part of that cost savings came from cutting dozens of administrative workers.

Scott Somers, a Mesa City Council member and Valley Metro's board chairman, called the strike unacceptable and "devastating" to riders who use bus service to get to work, school and medical appointments. "It's especially distressing in this economic condition," he said.

A strike in March 2012 crippled bus service in metropolitan Phoenix for six days before it was settled.