Hundreds of Newton teachers demonstrated during a rally outside City Hall Wednesday morning, demanding a new contract before the school year starts Tuesday.

Members of the Newton Teachers Association gathered because they are frustrated about the pace of contract talks, according to Michael Zilles, the union’s president.

More than a year ago, the union and the city negotiated a one-year contract extension to buy both sides more time to work out a new agreement. That extension expires Aug. 31, and teachers are concerned that a new deal is not in place.

“The message we’re sending is . . . ‘Working without a contract is no longer business as usual,’ ’’ Zilles said. “And we mean business about that.’’

On Twitter, photographs showed Newton Teachers Association members in their distinctive red shirts gathered on the lawn, many carrying handwritten signs with slogans supporting their cause.

“It is better to protest than to accept injustice,’’ read a sign carried by a woman at the rally, quoting Rosa Parks. “Newton teachers deserve a contract!’’

At one point, demonstrators broke into song, singing lyrics from a Twisted Sister song: “We’re not gonna take it, no, we ain’t gonna take it, We’re not gonna take it anymore!’’

In a statement released Wednesday morning, Mayor Ruthanne Fuller said she wanted to address the NTA members gathered at City Hall, but that Zilles rejected her offer.

With the contract expiring on Saturday, the sides have been “hard at work’’ trying to reach an agreement. She said the sides are negotiating in good faith.

“You need and deserve a fair contract now. I will do all I can to help settle this contract expeditiously and make sure NPS has the resources it needs,’’ Fuller said in the statement.

The rally came on the same day that is traditionally reserved to welcome back Newton teachers ahead of the new school year.

But this year’s opening-day ceremony at Newton South High School was canceled earlier this month by Superintendent David Fleishman, who cited declining attendance.

Fleishman instead said that the union will hold a meeting at the school. Zilles said that would mean NTA members would not be able to speak directly to city leaders about the contract.

The NTA is seeking a 12 percent raise over four years, changes in parental leave, and pay increases for behavior therapists, many of whom work outside their usual hours, Zilles said. The schools are offering yearly increases of about 2 to 2¼ percent each year, he said.

On Wednesday morning, about 500 people demonstrated at City Hall on Commonwealth Avenue, said Lieutenant Bruce Apotheker, a Newton police spokesman. Zilles estimated the turnout to be closer to 1,000 people.

Officers were on scene to handle traffic, Apotheker said. “From our end, it went smoothly,’’ Apotheker said.

During the school year, NTA members will also remain silent during some staff meetings to show they are unhappy with progress toward a new agreement and will wear union shirts during back-to-school nights with parents, Zilles said.

During those sessions, many members are expected to wear stickers that say “Fair contract now,’’ he said.

Zilles and Ruth Goldman, chairwoman of the School Committee, said in separate interviews that the sides are making progress, but the process has been slow. The sides met Monday and plan to meet again in September and October.

“There’s no question that the rally and the start of the school year put more pressure on the School Committee. And no one wants the contract settled more than we do. It takes a lot of time,’’ Goldman said.

“We want the teachers to have a good contract.’’

John Hilliard can be reached at john.hilliard@globe.com.