Mr. Strauss-Kahn, 62, who was widely expected to become the Socialist candidate for the French presidency, was apprehended by detectives of thePort Authority of New York and New Jersey in the first class section of the jetliner, and immediately turned over to detectives from the Midtown South Precinct, which covers the part of Manhattan where the hotel is, officials said.

The New York Police Department took Mr. Strauss-Kahn into custody, where he was “being questioned in connection with the sexual assault of a hotel chambermaid earlier this afternoon,” Deputy Commissioner Paul J. Browne, the department’s chief spokesman, said Saturday evening.

A spokeswoman for the Manhattan district attorney’s office said Mr. Strauss-Kahn had not yet been “formally charged” in the case and was not expected to be arraigned before a judge until later in the evening.

The arrest came at about 4:40 p.m., when two detectives from the Port Authority suddenly boarded Air France Flight 23, as the plane idled on the tarmac, said John P. L. Kelly, a spokesman for the agency.

“It was 10 minutes before its scheduled departure,” Mr. Kelly said. “They were just about to close the doors.”

Mr. Kelly said Mr. Strauss-Kahn was traveling alone and was not handcuffed during the arrest.

“He complied with the detectives’s directions,” Mr. Kelly said.

The Port Authority officers were acting on information from the Police Department, whose detectives had been investigating the assault of a female employee of the Sofitel New York, at 45 West 44th Street, in the heart of the theater district.

Mr. Strauss-Khan, a former French finance minister, had been expected to declare his candidacy soon, after three and a half years as the leader of the fund, which is based in Washington, where he was considered by many to have done a good job in a very difficult period of global economic strain, when the bank itself has become vital to the smooth running of the world and the European economy.

His time at the bank was tarnished in 2008 by an affair with a Hungarian economist who was a subordinate there. The fund decided to stand by him despite concluding that he had shown poor judgment in the affair. Mr. Strauss-Kahn issued an apology to employees at the bank and his wife, Anne Sinclair, an American-born French television journalist.

In his statement then, Mr. Strauss-Kahn said, “I am grateful that the board has confirmed that there was no abuse of authority on my part, but I accept that this incident represents a serious error of judgment.” The economist, Piroska Nagy, left the fund as part of a buyout of nearly 600 employees instituted by Mr. Strauss-Kahn to cut costs.

In the New York case, Mr. Browne said it was about 1 p.m. on Saturday when the maid, a 32-year-old woman, entered to clean Mr. Strauss-Kahn’s room.

“He came out of the bathroom, fully naked, and attempted to sexually assault her,” Mr. Browne said.

At some point during the assault, the woman broke free, Mr. Browne said, and “she fled, reported it to other hotel personnel who called 911. When the police arrived, he was not there.” Mr. Browne said it appeared that Mr. Strauss-Kahn left in a hurry. Investigators found his cellphone in the room, which he had left behind, he said.

He added: “We learned that he was on an Air France plane, and the plane was held and he was taken into custody.”

Mr. Browne said an Emergency Medical Services ambulance arrived at the hotel to take the maid to Roosevelt Hospital for what Mr. Browne described as treatment for “minor injuries.”

No matter the outcome of Saturday’s arrest, it will likely throw the French political world into turmoil and the Socialist Party into an embarrassed confusion.

Mr. Strauss-Kahn, a leading member of the party, has been considered the front-runner for the next presidential election in France in May 2012. Opinion polls have shown him to be the Socialists’ most popular candidate and running well ahead of the incumbent, Nicolas Sarkozy, who leads the center-right party.

France has been waiting for Mr. Strauss-Kahn to decide whether to run for his party’s nomination in a series of primaries, which would mean giving up his post as managing director of the fund.

The view in France was that if Mr. Strauss-Kahn wanted to run, he would have to make his intentions clear early this summer, and most politicians and analysts have been predicting that he would not be able to resist the chance to run the country.

Mr. Strauss-Kahn contested for the nomination five years ago, losing to Ségolène Royal, who ultimately lost a second-round runoff to Mr. Sarkozy. Mr. Sarkozy then arranged for Mr. Strauss-Kahn to get the I.M.F. job, partly to remove a popular rival from France’s political landscape.