Hundreds of janitors, with support from four members of San Francisco’s Board of Supervisors, marched downtown Thursday to bring attention to what they say are unfair wages and benefits paid by the cleaning companies that employ them.

The contract between Local 87 of the Service Employees International Union, which represents 3,500 janitors in the city, and a group of janitorial services that includes ABM Industries is set to expire Sunday. The two sides are in negotiations over a new contract, but are at odds over what it should look like.

At least 300 janitors marched in circles at A.P. Giannini Plaza at California and Kearny streets in the afternoon before taking to the streets of the Financial District, where many of the workers clean offices. Minor traffic delays resulted.

Seventeen people were arrested at First and Market around 5:30 for refusing to get out of the street during the protest.

“The purpose is to get the attention of the companies. We don’t want our streets to be all blocked,” said Alan Benjamin, a union spokesman. “We want to send a message. All we want is a fair deal. We want serious negotiations.”

The demonstration was part of a week of scheduled protests that will culminate Friday when union members are scheduled to vote on whether to go on strike, Benjamin said.

Management representatives for the janitorial companies could not be reached for comment.

The disputed contract covers about 85 percent of the union members who clean downtown office buildings, including Airbnb, Uber and The Chronicle, union officials said.

The workers are asking for across-the-board pay raises, including an increase in their current base pay of $13.45 an hour, without benefits, to $19.45 an hour. They also want to maintain a low co-pay on health care plans and decent dental coverage, an increase in what their employers contribute to their pension funds and a stronger employee grievance process, union representatives said.

Benjamin accused several of the janitorial services of reducing full-time employees to 7½ hours of work per day to avoid providing benefits.

“They want to change the rules of the game,” Benjamin said. “We’re saying that’s not right. This is a union town.”

Supervisors Malia Cohen, Aaron Peskin, London Breed and Scott Wiener attended part of Thursday’s demonstration to show support for the janitors.

“I’ve worked with the janitors of Local 87 for a long time,” Wiener said. “These are some of our lowest-paid workers. And they deserve a living wage.”

Jenna Lyons is a San Francisco Chronicle staff writer. Email: jlyons@sfchronicle.com Twitter: @JennaJourno