When Mayor Thomas M. Menino (D) welcomes delegates to the Democratic National Convention here, the chairman of the California delegation plans to stand up and walk out.

Art Torres has asked the rest of his state's 541 delegates, alternates and committee members to leave with him as a show of solidarity with Boston police and firefighters, who are locked in a bitter contract dispute with Menino that threatens to disrupt the convention. "I don't know what other message to give to this mayor that workers who protect our lives deserve a contract," Torres said.

A week before 35,000 people begin arriving here, organizers insist the city is ready for the July 26 opening. Bumpy roads have been smoothed, lampposts painted, parks manicured. Homeland Security Secretary Tom Ridge approved a vast security apparatus that will turn the convention site into a fortress and leave residents grappling with traffic jams, closed roads and other inconveniences.

But concern is growing that the standoff between the unions and City Hall will not only embarrass the Democrats but also disrupt events inside and outside the convention hall and serve as a distraction for emergency workers that will compromise the safety of attendees at a time of heightened national alert.

The more than 1,400 members of the Boston Police Patrolmen's Association and Boston Firefighters Local 718, plan to protest every convention event the mayor hosts or attends. Their efforts will focus on the walkout during Menino's opening speech July 26 and on picket lines outside the 29 official state welcoming parties at high-profile venues likely to draw big Democratic names.

The unions have written to all the state delegations asking them not to cross their picket lines. The leaders of at least six -- North Dakota, Maine, Missouri, Ohio, Tennessee and California, the nation's largest -- have said they will avoid picketed events and will encourage their delegates to do likewise, which has led organizers to contemplate cancellations. Ohio delegation chairman Dennis L. White wrote the police union this week that he and his staff are prepared to join the walkout on Menino.

The police will also protest in City Hall Plaza on July 26, and smaller demonstrations are planned throughout the week.

National conventions for both major parties have experienced protests from activist groups with varying motivations -- most notably when police clashed violently with war protesters outside the Democrats' convention in Chicago in 1968 -- but rarely has organized labor become involved.

Boston convention chief executive Rod O'Connor has tried to tamp down concerns, telling reporters this spring that some unions reached contracts with Los Angeles just three weeks before the 2000 Democratic convention opened there.

"This is something that unions occasionally threaten because they have such leverage within the party," said Colorado College professor Robert Loevy, a historian of political conventions. "But to the best of my knowledge, this could be the first time that it wasn't settled by the time the event begins."

The dispute has disrupted construction at the FleetCenter, where the convention will be held. Last month several vehicles were turned away by shouting pickets, and work was stalled for days.

Sen. John F. Kerry (Mass.), who the convention will nominate as the Democratic Party's presidential candidate, canceled a June 28 speech to the U.S. Conference of Mayors, hosted here by Menino, when union members made clear they would be picketing the event. "I don't cross picket lines. I never have," Kerry said at the time.

About 12 percent, or 500 of the 4,352 delegates to the convention, are union members, according to the AFL-CIO. The top brass of the AFL-CIO will be in Boston and will not cross the lines, said Lane Windham, spokeswoman for the AFL-CIO.

But the labor federation does not want the convention's events to be interrupted. AFL-CIO President John J. Sweeney has spoken to the police and firefighters' unions and to the mayor, Windham said, in an effort to resolve the labor dispute.

But if resolution is not reached, Jeff Zack, a spokesman for the Firefighters International Union, said some union-sponsored events that Menino planned to attend might be postponed or canceled.

Concerned about that possibility, a number of women- and minority-owned businesses doing work for the convention warned Friday at a news conference that any picketing would be damaging to them.

"The truth of the matter is: By boycotting these parties, they really are hurting some of the core people who are part of the Democratic Party," said Colette Phillips, whose communications company is planning a $100,000 jungle-themed party for the California delegation in the Franklin Park Zoo.

Karen Grant, a spokeswoman for Boston 2004, the city's host committee, said no events had been canceled. She added that contracts with vendors will be honored.

Several state delegations said they are still deciding how to handle the labor dispute.

Kentucky Democratic Party executive director Eddie Jacobs said Friday that he and party chairman Bill Garmer will not cross picket lines, and that they will let their 72 delegates, alternates and committee members decide for themselves how to proceed.

Vern Thompson, executive director of the North Dakota Democratic Party, said that instead of his state's official welcome party, he will ask delegates to attend a union party, dubbed "Boston Police and Firefighter Heroes Welcome the DNC," in Dorchester, a neighborhood south of downtown Boston, on Sunday, July 25.

Jim Barry, a police union spokesman, said he expected as many as 1,000 delegates to attend the union-hosted reception, and that his members would be joined on their picket lines by union members from across the country who have pledged to travel to Boston. "We want the delegates here," he said. "It's not them we have a problem with; it's the mayor."

For Menino, who worked to bring the convention to Boston, these have been trying months. His appearances have been picketed by union members since March. "This is a safe haven right now," he said, only partly in jest, at a recent private reception introducing an advertising campaign to promote the city for the convention. "Anybody out there got a sign?"

The city has offered the unions an 11.9 percent raise over four years, and Menino has said that while the convention is important to him, he will not mortgage the city's future by signing a contract he says it cannot afford. Union leaders say they are holding out for a 17 percent raise.

Three months ago, more than 30 city unions were without contracts, but Menino has since signed agreements with 75 percent of the city's workforce. But negotiations with the police, who rejected Menino's repeated calls for arbitration, have been acrimonious.

Aides to the mayor have accused the union, which supported George H.W. Bush (R) over home-state Gov. Michael S. Dukakis (D) in 1988, of playing politics. The union said it will probably endorse Kerry, but not until it has a contract. Unlike many unions in Massachusetts, the Boston police association has donated money to both political parties, not solely to Democrats.

Both sides of the labor dispute have been meeting with the state's Joint Labor Management Committee, a mediation body, which was seeking to broker an agreement and had voted to appoint an outside arbitrator.

But this week, Gov. Mitt Romney (R) intervened by replacing the acting head of the management committee, citing a concern that the dispute would not be resolved before the convention. "Public safety personnel during the Democratic National Convention should be focused entirely on security, not manning a picket line," he said.

The two sides have been called to a meeting with mediators Monday, leading to some optimism that a deal might be possible before the convention.

"A road map for the settlement of this dispute is now in place, so I don't see the point of more protests," said Seth Gitell, Menino's spokesman. The mayor has been working the phones with various state delegations, Gitell said, hoping to convince them that the city is doing all it can to find a solution.

As of now, he said, Menino's convention week schedule remains unchanged.

Birnbaum reported from Washington.

At the FleetCenter in Boston, the newly paved area is to accommodate parking, foreground, and provide a "demonstration zone."