Parties in the bitter Port Authority labor dispute last night reached a tentative agreement on a new contract at International AFL-CIO headquarters in Washington, D.C., after four days of special talks unprecedented in their 44-year history.

The announcement came around 7:30 p.m. in a joint statement issued on behalf of the authority and Local 85 of the Amalgamated Transit Union, representing 2,300 bus-trolley workers and first-level supervisors.

It said the two sides "are very pleased to announce that we have reached a tentative contract agreement. We are glad to inform the community that Port Authority public transportation service will continue without interruption."

The breakthrough removes the uncertainty for commuters who had been bracing for a transit work stoppage that could have occurred as soon as Monday, when the authority was set to impose a contract, which could have resulted in a work stoppage.

Local 85 employees will continue working, however, under terms of the previous contract that expired June 30 until a new contract is ratified by the union and approved by the Port Authority board.

Terms of the new contract were not disclosed, in keeping with the secretive nature of the meetings and negotiations that have been taking place over the last four days under a general news blackout.

It is believed that the authority's representatives, Chief Executive Officer Steve Bland and its negotiator, Michael Palombo, of Campbell, Beatty & Durant, regularly updated County Executive Dan Onorato of developments, because Mr. Onorato has insisted that long-term legacy costs be brought under control.

Without that, Mr. Onorato vowed not to release $27.7 million in revenue from county drink and car rental taxes. The authority has said it needs the funding to keep from running out of money by the middle of next month.

"I am pleased to hear that leaders (of the union and authority) have reached a tentative contract agreement and that transit will continue without interruption," Mr. Onorato said in a statement released last night.

Local 85 President-Business Agent Patrick McMahon and the union's chief labor counsel, attorney Joseph J. Pass, said terms of the tentative agreement will be presented to the membership for review and proposed for a vote on a date to be determined.

Following union ratification, the authority board is to call a special meeting to review the agreement and give final approval.

The tentative agreement nullifies the authority's plans to impose a contract that it previously adopted and that was to go into effect at 12:01 a.m. on Monday.

Local 85 has maintained the contract was illegal and constituted a lockout.

Some observers have speculated that the AFL-CIO and the International ATU, one of the federation's union members and Local 85's parent organization, got involved because an imposed contract -- if it stood up in court -- could pose national implications, establishing precedent for private companies and other public entities to do the same as the Port Authority.

Warren S. George, a former Local 85 official and a former Port Authority employee, who is now president of the International ATU, was said to be largely responsible for bringing the feuding parties to Washington.

"He wanted to do all he could to keep a strike or lockout from happening," ATU spokesman Shawn Perry said earlier yesterday. "He's playing a central role, intervening. Other than that, there's a blackout and it includes me."

Mr. George, 75, has been president of the international transit union since July 2003. He had been a bus driver with Critchlow Bus Lines before it was folded into the Port Authority in 1964, where he worked as a mechanic. He's a former Local 85 president and financial secretary who retired in 1992.

Richard L. Trumka, another top union official with Pittsburgh area roots, was said to have participated in the negotiations. Mr. Trumka, a native of Nemacolin, Greene County, has been secretary-treasurer of AFL-CIO since 1995 after being involved on some high profile labor battles as president of the United Mine Workers America.

Union officials and some of their top financial and negotiating experts employed "shuttle diplomacy" for most of the four days, starting Saturday and Sunday, with detailed reviews of authority finances, including budget projections, health care and pension costs.

Meetings that began early in the morning lasted late into the night, although at times Mr. Bland and Mr. Palombo would sit for long periods of time waiting for negotiators to return with information, suggestions or questions.

Mr. Perry said he thought moving the negotiations out of Pittsburgh and away from news media and public pressure in Pittsburgh helped bring about a positive result.

"Everybody was tired of seeing the contract negotiated in the press" and on talk radio, he said.

The two sides officially started negotiations for a new agreement last September with an exchange of letters identifying participants and outlining procedures. Face-to-face meetings didn't start until earlier this year, but Mr. Bland attended only one of them.