LBANY, June 20 — Senate Republicans forced through a measure today that would allow landlords to take thousands more New York City apartments off rent regulation in the next eight years, outmaneuvering Democrats who had sought to prevent a further erosion in rent protections for tenants in a million units.

A stunning night of political power plays in the Capitol ended with the State Legislature's renewal of the expiring rent laws in a way that strengthened the power of landlords to charge market rents for more of their apartments and left intact rules that have led to the deregulation of thousands of units.

Led by Joseph L. Bruno, the Senate majority leader, Senate Republicans introduced and passed a bill to extend the rent laws shortly after they expired just after midnight.

The bill included provisions that were described by some legislators and tenants' groups as favorable to landlords and repugnant to tenants. One Democratic senator, Eric T. Schneiderman, called it a "declaration of nuclear war on rent-regulated tenants in New York."

Senate Republicans and George E. Pataki, the Republican governor, engineered the surprise move to introduce their own bill, which codified existing regulations that allow landlords to start charging market rates for vacated rent-stabilized apartments once the rent reaches $2,000 a month even if they later charge less.

While these Republican leaders said their changes would not immediately result in more deregulated apartments, tenants' groups argued otherwise. It was a major blow for the groups, who had sought to revise the rent laws to include more tenant protections.

After passing the bill, the Senate Republicans hastily dispatched it to the Assembly and filed out at daybreak to adjourn for the summer. Mr. Bruno, who had vowed to end the legislative session on Thursday, said he was done talking and would not revisit the issue.

Because the New York Legislature does not have a procedure to resolve conflicting bills passed by the two chambers, the Assembly Democrats were left with a difficult choice. They could accept a bill that could deregulate thousands of apartments whose rents creep up beyond the $2,000 mark — more than 300,000 apartments over the next decade, according to estimates by tenant organizers. Or they could allow the rent law, which technically expired at 12:01 a.m. today, to fade away entirely.

Assembly Speaker Sheldon Silver, who had wanted to raise the rent level at which apartments become deregulated and add additional protections for tenants, angrily accused Governor Pataki and Senate Republicans of mounting a "sneak attack" to gut the rent-regulation system in New York.

"There's only one word for all of it: shameful," Mr. Silver said. "I vow to you that this conference will carry on the fight beyond today for the tenants of this state."

The Assembly Democrats, still in session this evening, passed the Senate bill shortly before 8 p.m. by a vote of 106 to 38. Members said they were reluctant to walk away from the rent laws entirely, or to gamble that the Senate would eventually come back for more negotiations. So they called it quits in what amounted to a weighty game of political chicken.

From the start, Republican leaders made it plain that the intent of their bill was to erode what they say is a system of government price-fixing that has outlived its usefulness.

The system covers about a million rent-stabilized and 60,000 rent-controlled apartments in New York City, whose rents are kept at below-market levels to ensure there is a supply of apartments available for working-class and poor New Yorkers. There are about 2 million rental units in the city. Tenants' advocates argue that the rent laws are the only protections left for families who have gradually been priced out of their own city.

Governor Pataki immediately signed the bill, which was effective retroactive to 12:01 a.m. today.

"This is a very good, common sense approach," the governor said today, adding that the changes would reinforce a rent system that works well already.

The new law strengthens existing regulations that make it easier for landlords to deregulate apartments in the long run, and also further limits the ability for New York City to influence the state's rent laws. Under the new law, the city will be allowed to make decisions only on renewing its own city rent laws and deregulating certain categories of apartments.

Lobbyists for the tenants' groups seemed exhausted by the swift turn of events, and could do little more than mourn what they saw as a lost opportunity to safeguard the rent system for future generations. In particular, they had wanted the repeal of high-rent vacancy decontrol, the provision that allows landlords to charge market rents for vacant apartments after the monthly rent of a regulated apartment reaches $2,000 a month.

"There can be no doubt that the purpose of this bill is the final destruction of the rent laws over the next eight years," said Michael McKee, associate director of New York State Tenants and Neighbors Coalition, an advocacy group. "Their tactic of unveiling the deregulation plan at the last minute, in the dark of night, is reprehensible."

But Senate Republicans and several landlord groups stressed that the bill put forth largely technical changes that would clarify the existing mechanism for deregulating rent-stabilized apartments, rather than create any new ones.

Almost all people in rent-stabilized or rent-controlled apartments are permitted to keep their rent protections for as long as they keep their homes.

If they die, their immediate family members can inherit the protections with the apartments, provided they have lived in them. Six years ago, state lawmakers adopted amendments that let landlords deregulate vacant units if they get the legal rent up to $2,000 a month.

The law, known as high-rent vacancy decontrol, allows landlords to charge any rent the market will bear after the apartments are deregulated.

"No one is in jeopardy," Senator Bruno said. "Not one more unit is placed in decontrol, not one."

Several advocates for landlord groups said that while they would have preferred legislation rolling back the rent laws, and allowing market forces to become more of a factor in New York, they were not unhappy with the way things turned out.

"We are glad that in fact the status quo has been maintained," said Joseph Strasburg, president of the Rent Stabilization Association, a major owners' group.

New York's rent laws date back to World War II, in one form or another, and the current system of rent stabilization evolved from a 1974 state law, known as the Emergency Tenant Protection Act. The law applied to New York City, and to any municipality in Westchester, Rockland and Nassau Counties that chose to opt into it. Nearly all of the affected units are in New York City, however.

After weeks of closed-door negotiations, Albany's leaders remained deadlocked on the rent laws right until the end. The laws, which were originally set to expire last Sunday, had to be extended four times by special legislation — for one day each time — to keep them from lapsing.

For most of today, many tenants scrambled desperately to find out what was happening in Albany.

One tenant leader, Dave Powell, director of organizing for the Metropolitan Council on Housing, said that tenants would continue to fight for the city's right to have more say over the fate of its housing.

"Perhaps we were naïve to think that we could rely on any justice for tenants from the Legislature, the Assembly included," he said. "We're painted into a corner at this point. It's do or die."