A federal judge Thursday directed U.S. marshals to monitor an intensifying labor standoff between city employees and City Hall that has interrupted construction at the site of next month's Democratic National Convention.

For three days, hundreds of pickets representing unions working without a collective bargaining agreement have strolled the sidewalks outside Boston's FleetCenter, where the event will be held, declaring that with "no contract," there would be "no convention."

Only a smattering of workers got started this week. The sports arena must be equipped to be a political showcase, complete with speaker's platform and media facilities, by the July 26 start date. Several vehicles delivering supplies and equipment were turned away when confronted by shouting pickets.

City officials filed a motion Thursday complaining that members of the Boston Police Patrolmen's Association (BPPA) had violated a court order not to block access to the arena. District Court Judge Joseph Tauro said federal marshals would oversee the demonstrations and report back to him regularly.

"This is no ordinary labor dispute," said Tauro during an emergency court session. "I'm hoping that this, as the song goes, is a new beginning."

Labor disputes are hampering preparations by both major political parties as this summer's presidential nominating conventions approach. Unions are seeking leverage for better contracts and city officials are trying to ward off embarrassing disruptions to the high-profile events. In New York, where Republicans will gather in August, several unions that have worked for years without a contract or a raise applied for permits to protest during convention week.

Tens of thousands of workers poured into downtown Manhattan across from City Hall this week in a show of strength.

"Someone should ask the White House if they have concerns of the heroes of 9/11 standing outside Madison Square Garden, protesting that they can't get a contract from a Republican mayor when the president is trying to get reelected," said Stephen J. Cassidy, president of the Uniformed Firefighters Association.

Some Empire State workers have offered to join protesters in Boston. However, there are no plans to picket in New York, and a spokesman said Mayor Michael R. Bloomberg (R) is not concerned that the convention will be disrupted.

The long-simmering labor unrest in the Massachusetts capital -- which pits a collection of public unions led by the head of the police patrolmen, Thomas J. Nee, against popular Mayor Thomas M. Menino (D) -- is more pressing because Democrats rely heavily on organized labor for logistical and financial support in the November elections.

"This represents the other side of the Democratic Party reaping the bounty of their relationship with organized labor," said Paul Watanabe, a professor of political science at the University of Massachusetts at Boston. "With so much at stake and so much pressure on both sides, it's hard to believe something won't get done. But a similar situation for Republicans in New York does not have quite the same sting."

U.S. Marshal Anthony Dichio visited the FleetCenter on Thursday and met with city and union officials. He said his priority would be to "stop the entrances from being blocked, so that work can get done."

Nee denied the charges, saying that his members were exercising free speech and that they would demonstrate until an agreement is reached. The BPPA has rejected arbitration and is reportedly seeking a pay raise of about 17 percent over four years; the city has offered 12 percent. The BPPA supported Republicans in several recent elections.

State and national Democratic officials have expressed concern. Menino, who worked tirelessly to bring the convention to Boston, discussed the situation with Democratic National Committee Chair Terrence R. McAuliffe on Tuesday, and briefed Sen. Edward M. Kennedy (D-Mass.) on the proceedings Thursday.

"We hope to resume full construction activities at the FleetCenter tomorrow in order to fulfill our contract," said Thomas Goemaat, president and CEO of Shawmut Design and Construction.

City unions in Boston and New York cannot legally strike. Although several agreements have been reached in Boston in recent weeks, and others are said to be close to completion, 20 unions representing roughly 39 percent of Boston's workforce still lack contracts.

At least one state party, from Maine, has said its delegates will not cross a picket line if the dispute is not settled when the convention begins.

Garcia reported from New York.