NEW YORK — Commuters trudged through the freezing cold, rode bicycles and shared cabs Tuesday as New York's bus and subway workers went on strike for the first time in more than 25 years and stranded millions of riders at the height of the Christmas rush. A judge slapped the union with a $1 million-a-day fine.

The sanction was levied against the Transport Workers Union for violating a state law that bars public employees from going on strike. The city and state had asked that the union be hit with a "very potent fine."

"This is a very, very sad day in the history of labor relations for New York City," State Justice Theodore Jones said in imposing the fine.

The union said it would immediately appeal, calling the penalty excessive.

The strike over wages and pensions came just five days before Christmas at a time when the city is especially busy with shoppers and tourists.

The heavy penalty could force the union off the picket lines and back on the job. Under the law, the union's 33,000 members will also lose two days' pay for every day they are on strike, and they could also be thrown in jail.

The courtroom drama came midway through a day in which the walkout fell far short of the all-out chaos that many had feared. With special traffic rules in place, the morning rush came and went without monumental gridlock. Manhattan streets were unusually quiet; some commuters just stayed home.

The nation's biggest mass-transit system ground to a halt after 3 a.m., when the union called the strike after a late round of negotiations with the Metropolitan Transportation Authority broke down. The subways and buses provide more than 7 million rides per day.

New Yorkers car-pooled, shared taxis, rode bicycles, roller-skated or walked in the freezing cold. Early morning temperatures were in the 20s. Mayor Michael Bloomberg joined the throngs of people crossing the Brooklyn Bridge by foot.

"Hey, can I get a ride?" Jay Plastino asked a neighbor near his home in the northern tip of Manhattan. Plastino, who was headed to his midtown job, was angry at the union: "This is a big city. Don't they realize that?"

By Tuesday's evening rush hour, crowds were thick at both Penn Station and Grand Central Terminal as commuters waited for trains on the two suburban rail lines, where ridership had soared earlier in the day. The Long Island Rail Road, operating out of Penn Station, carried 50,000 passengers above its usual 100,000.

Gov. George Pataki said the union acted illegally and "will suffer the consequences." But union attorney Arthur Schwartz accused the MTA of provoking the strike.