Commuters spent the day on edge Monday as negotiators struggled to avert New York's first citywide bus and subway strike in more than 25 years, a walkout that could paralyze the big city at the very height of the Christmas rush.

Turning up the pressure on the city's transit agency, union members at two private bus lines in Queens walked off the job early Monday, and the union president warned that a full-blown strike was set to begin at 12:01 a.m. Tuesday.

More than 7 million daily riders would be forced to find new ways to get around if the 33,000-member Transport Workers Union shut down the nation's largest transit system.

Union President Roger Toussaint sounded pessimistic about reaching a deal as he appeared before a boisterous gathering of union members Monday evening.

"As we stand right now, with six hours to go until our deadline, it does not look good," he said. "I'm going to leave you now and go back to the hotel and give it one last shot."

MTA Chairman Peter Kalikow told New Yorkers to "keep your fingers crossed."

Meanwhile, the union posted a strike plan on its Web site, instructing members to lock up facilities safely and document everything they do to prevent "management sabotage."

If the union's executive board calls for a walkout, buses will drop off all passengers and return to their depots. Subways will finish their trips as turnstiles are chained and locked.

Mayor Michael Bloomberg said a walkout could cost the city as much as $400 million a day — a figure that includes police overtime and lost business and productivity. It would be particularly harsh at the height of the holiday shopping rush.

The mayor said a strike would freeze traffic into "gridlock that will tie the record for all gridlocks."

Transit workers are barred under state law from going on strike. A walkout could bring punishing fines.

The workers' old contract expired early Friday, but the union and the Metropolitan Transportation Authority agreed to keep talking through the weekend.

Many commuter were fed up with the uncertainty.

"Enough is enough," said Craig DeRosa, who relies on the subway to get to work. "Their benefits are as rich as you see anywhere in this country and they are still complaining. I don't get it."

The two sides were divided over wages and an MTA proposal to raise the age at which new employees become eligible for a full pension from 55 to 62.

More than 100 employees of the striking Jamaica Buses and Triboro Coach bus lines formed picket lines early in Queens, many chanting, "No contract, no work!"

Later in the day, hundreds of union members rallied outside Gov. George Pataki's office in midtown Manhattan, partly blocking traffic and screaming for a transit system walkout. "Shut it down!" they chanted.

The companies serve about 50,000 commuters, and are in the process of being taken over by the MTA, meaning they are not yet covered by a state law that prohibits public employees from striking.

The last citywide bus and subway strike in New York was in 1980. The walkout lasted 11 days.