BOSTON (AP) Hundreds of union pickets sympathetic to a police labor dispute on Tuesday surrounded the site of the Democratic National Convention, delaying the start of preparations for the political gathering in July.

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

The picketing, scheduled to continue around the clock, 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 already have 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, to hand out leaflets critical of Menino. 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. "Solidarity is alive and well in Boston."

Menino, who worked to bring the convention to his city, showed no sign of relenting. He said his legal team was exploring whether the protest violated an agreement between FleetCenter management and the police union to limit the number of picketers who will be on the property through July 23, the Friday before the convention begins.

"We’re looking at crunch time, but I have an obligation to the citizens of Boston as well," the mayor said. "I’m not going to mortgage my city’s future."

Menino said he had not been in recent contact with the Democratic National Committee about the union troubles, nor had hespoken with Massachusetts Sen. John Kerry, the party’s presumptive presidential nominee.

Kerry said Tuesday that he was "sure everything will work itself out."

Massachusetts Sen. Edward Kennedy issued a statement calling for "an all-out effort to reach an agreement that’s fair for the workers and fair for the city."

The Democratic National Convention Committee, which is organizing the event, declined comment Tuesday.

Faced with crossing the picket line Tuesday, 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, 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. Unless the trade unions agree to cross the picket line, convention planners could be forced to hire nonunion workers, an unthinkable prospect for a Democratic Party built on a foundation of organized labor.

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