SAN FRANCISCO -- Thousands of union janitors snaked their way through downtown San Francisco in a noisy protest Wednesday afternoon, snarling Financial District traffic.

Carrying signs reading "standing up for working families," and "janitors are the 99%," demonstrators marched down Market Street, blowing vuvuzelas, banging kettle drums and blocking westbound traffic on the busy thoroughfare.

(SCROLL DOWN FOR PHOTOS)

Organizers report 27 people were arrested during the protest.

This march is the second large protest this week by the group, which has been locked in heated contract negotiations with janitorial services contractors operating in more than 400 buildings -- mostly located in the same downtown core as the demonstrations.

Late last month, the contract between some 3,000 union janitors represented by Service Employees International Union Local 87 and a handful of janitorial service providers ran out. Negotiations have stalled over health care, with union representatives unwilling to accept a new contract that would increase worker co-pays up by as much as $600 per month.

"The managers' demands are ridiculous, given that San Francisco has the strongest real estate market in the entire country," union spokeswoman Cecille Isidro told The Huffington Post.

San Francisco's social media-fueled tech boom has supercharged the city's commercial real estate market with skyrocketing rents.

"We clean the buildings of some of the country's biggest and wealthiest corporations: the very ones that are standing in the way of working people being able to provide affordable healthcare for our families," janitor Mohamed Ismael said in a statement. "For the past 18 years, I've worked hard and sacrificed so that I could provide healthcare for my children. Now the companies want to take it all away."

While the union has led multiple protests, there has yet to be a work stoppage. Union members, however, have voted to give negotiators the ability to call a strike.

"Stop cleaning their offices," exhorted a Teamsters union representative, who joined Wednesday's march in solidarity. "Let them take out their own trash for a while and then see what they offer."

If a janitors' strike does occur, it would be the first in San Francisco's recent history. A week-long strike was called in Silicon Valley in 2008.

Representatives from the janitorial contractors did not immediately respond to requests for comment.

Similar labor unrest has flared over health care issues elsewhere in San Francisco recently. Earlier this week, members of a union representing employees at two of the city's ionic fine art museums voted to authorize a strike over management's demands of increased health care costs.

In May, union employees working on the Golden Gate Bridge threatened to join with the Occupy movement and block morning rush-hour traffic across the iconic span, protesting an effort to shift a larger portion of health care costs onto workers.

Update: In a statement to the Huffington Post, Jim Beard, lead negotiator for the San Francisco Maintenance Contractors Association, said "We are negotiating in good faith, committed to reaching an agreement that is fair for all interested parties, including both our employees and our customers. We firmly believe that we can reach such an agreement."

Check out photos from Wednesday's demonstration below:

Loading Slideshow...