Hundreds of union pickets sympathetic to a police labor dispute surrounded the site of the Democratic National Convention (search) on Tuesday, preventing preparations for the political gathering to begin in less than two months.

Union firefighters, electricians and other trade workers joined police officers picketing over a long-simmering dispute with Mayor Thomas Menino (search). The 1,400-member police union has been without a contract for two years. Talks broke down on Monday, with each side blaming the other for the impasse.

The dispute could extend to the party itself. The head of Maine's Democratic Party said she thinks her state's delegates — many of whom are union members — will refuse to cross the picket lines during the convention.

"The Maine Democratic Party has very close ties to organized labor," chairwoman Dottie Melanson said. "A great many members of our delegation would absolutely not cross a picket line."

The beginning of round-the-clock picketing coincided with the start of a $14 million construction project to prepare the FleetCenter, a sports arena, for the Democratic National Convention on July 26-29. Telecommunications workers have already said they won't cross police pickets to install thousands of miles of telephone and data lines.

Pickets gathered at the city's North Station commuter site, which shares a building with the FleetCenter, handed out leaflets critical of Menino to people on their way to work. Some held signs reading, "Friends Don't Let Friends Cross Picket Lines."

"The message is being sent that we're serious," said Thomas Nee, president of the Boston Police Patrolmen's Association (search). "Solidarity is alive and well in Boston."

Faced with crossing the picket line, many subcontractors reporting for work turned back instead, including a fleet of 18 moving trucks driven by Teamsters.

"We've been Teamsters for 43 years," said Ed Owens, president of Owens Movers. "We don't cross picket lines. Our guys were excited. It's the biggest move in Boston. It was disappointing."

The Greater Boston Labor Council (search), which represents 90,000 workers in 93 unions in the area, on Monday rejected a project labor agreement that promised no union strikes if convention organizers used only unionized labor on construction projects at the FleetCenter. Organizers could be forced to hire nonunion workers, an unthinkable prospect for a Democratic Party built on a foundation of organized labor.

A spokeswoman for convention organizers declined to comment. Michael Kineavy, director of Menino's office of neighborhood services, said the city expected union workers to begin the overhaul as planned.

"We've had conversations with the people doing the work, and our expectation is they will show up and get it done," Kineavy said.

Dennis DeMarzio, the city's chief operating officer, said the patrolmen's union twice has refused an offer to settle the contract through binding arbitration. He said the union's financial proposal is "far and away in excess of any contracts the city has agreed to, including the teachers."

Richard Rogers, head of the labor council, said the vote showed area unions are behind the police.

"If I'm head of the DNC, if I'm Terry McAuliffe (search), I'm thinking maybe it's time to get involved and push this process along," Rogers said of the city-police dispute. "In my mind, it sends a very strong message to the mayor that he needs to get back to the table and resolve this."