A handful of workers crossed police picket lines Wednesday to begin preparing the downtown sports arena for the Democratic National Convention (search), but the majority of tradesman stayed away as union protests continued to pose problems for convention organizers.

Labor troubles became more than just an image problem for Democrats when union workers on Tuesday surrounded the FleetCenter on what was to have been the first day of the arena's seven-week, $14 million overhaul.

Hundreds of firefighters, electricians and other trade workers stood beside unionized police officers, who organized the demonstration to protest their lack of a contract. The union has threatened to maintain a round-the-clock presence outside the FleetCenter and the attached North Station commuter center until Mayor Thomas Menino (searchmeets its contract demands.

The work never began on Tuesday. Most subcontractors refused to cross the picket line — to do so is a cardinal sin of organized labor — and those who did cross were greeted with shouts of "scab" and "shame on you."

Tom Goemaat, president and chief executive of Shawmut Construction, general contractor for the project, said about a dozen laborers entered the building Wednesday morning to begin work on the floor.

But the Teamsters of Owens Movers — a key subcontractor for the early stages of the conversion — refused to the cross the picket lines, Goemaat said. A truck carrying concrete workers who were to pour the foundation for a temporary media facility next to the FleetCenter was turned away by the picketers.

"We totally respect police and firefighters' right to protest. We're just trying to do our job," Goemaat said. "It's getting very tough, but it was always tough. We're hoping it gets resolved quickly."

Police Commissioner Kathleen O'Toole, who visited the picketers Wednesday morning, said she was optimistic that an accord could be reached with the patrolmen's union before the convention begins July 26.

Menino, who worked tirelessly to bring the four-day national convention to Boston, showed no sign of relenting, despite the threat the police union dispute poses to the preparations for the gathering and the city's reputation as a convention city.

"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."

Menino has offered police an 11.9 percent raise over four years and sent the union a letter Monday requesting expedited arbitration. But the union, which wants a 16.5 percent increase, said the process would inevitably leave them with less than what the mayor has given other city unions.