BOSTON—Demonstrators crowded Boston’s Tremont Street yesterday evening in expectation of the gubernatorial debate between Republican candidate W. Mitt Romney and Democrat Shannon P. O’Brien—the last debate scheduled before the Nov. 5 election.

Chanting supporters of the gubernatorial candidates shared the crowded sidewalks with activists representing special-interest groups in a rally that dissipated abruptly when the debates began.

Trucks covered in O’Brien and Romney posters paraded before the Suffolk University Law School building, where the debates were held.

A demonstrator leaning over the curb sounded out-of-tune blasts on a large trumpet as a subwoofer beat shook an O’Brien truck with rhythmic thumps.

“Yeah, O’Brien!” he cried between breaths.

Large television screens on each side of another O’Brien truck showed videos of a bull being dragged to the ground at a rodeo. An L.E.D. display on its front read “Shame on Mitt Romney,” while a placard saying “Cruelty is not the Olympic spirit” was affixed between its tail lights.

Boston educator Kerry A. Costello stood on the curb and shouted contentiously at Romney vehicles as they passed. She participated in the demonstration, she said, because she felt that a Democratic governor would improve the Commonwealth’s educational system.

“I’m a teacher and a public activist and a woman and it’s time we got pretty boy out of here,” she said, referring to Romney.

“Teach kids English. Teach Mitt math,” read a handwritten poster not far from her.

Robert Sneirson from Brookline held a “Disability Community for O’Brien” sign in his wheelchair on the street corner.

“We worry how Romney can cut all of his taxes without hurting services for persons with disabilities,” he said.

“We are here to give notice that we won’t take this sitting down.”

Romney activists leaned from the windows of poster-covered buses in a parade led by a decorated truck that looked as though it might once have served ice cream. A sound system on its roof projected the repeated cry, “We want Mitt!”

At one point during the parade, an O’Brien protestor stepped into the street, snatched a Romney sign from someone in one of the buses, and snapped the wooden dowel attached to it in half.

“I’m surprised about how violent people are here,” said Mike A. Ginsburg, a senior at Groton-Dunstable High School who had come to support Romney. “It’s not that big a deal.”