Chaos loomed in Boston yesterday as hundreds of union workers hit the picket lines and prevented the start of construction preparations for this summer's Democratic National Convention.

Seven weeks before Democrats gather to nominate hometown son Sen. John Kerry, city employees lined up at the Fleet Center, the site of the convention, to voice their displeasure with Mayor Thomas Menino over a labor dispute that has left the city's largest police union without a contract for two years.

"The message is being sent that we're serious," said Thomas Nee, president of the 1,400 member Boston Police Patrolmen's Association. "Solidarity is alive and well in Boston."

Electricians, firefighters and other union workers joined Boston cops in handing out leaflets mocking Menino as round-the-clock picketing began, one day after negotiations between cops and City Hall broke down.

The picketing prevented the start of a $14 million construction project slated to transform the Fleet Center for the convention, which runs from July 26-29.

Trucks driven by Teamsters were turned around when drivers saw the picketers, and a slew of other employees refused to cross lines and go to work.

But Menino didn't show any intention of giving in to union demands.

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