Police tonight arrested a man believed to be a Palestinian who attempted to plant a bomb aboard an El Al jet at Heathrow Airport yesterday.

The bomb was concealed in the baggage of the suspect's girlfriend, who was to have traveled on her own to Israel.

The man, identified as Nezar Hindawi, a 35-year-old Jordanian citizen, was arrested at a west London hotel where he was said to have checked in this morning. A terse police statement said only that Hindawi, who was being held without charge under Britain's Prevention of Terrorism Act in a high-security police station, had been located through "information received" in a tip, and had offered no resistance to arrest.

The manager of the London Visitors' Hotel, Naim Oran, said he recognized Hindawi from a photograph distributed by police last night and telephoned Hindawi's brother, whom he said he knew. The brother, Oran said, talked Hindawi into turning himself in and the police were called.

Earlier today, Hindawi was reported to have been in Britain at least four years ago, leading to speculation that he may have been a terrorist long awaiting an opportunity to act. Additional concerns were raised by the suggestion that he may have been part of some extremist group whose other members remain at large.

The editor of Al Arab, Europe's biggest Arabic daily newspaper, said Hindawi had sought a job there as a journalist in 1982. He was hired as a copy aide, then fired after two months when he proved "belligerent and arrogant."

Details of his movements since then remained sketchy. He reportedly met his Irish girlfriend, Anne Marie Murray, a chambermaid at the London Hilton Hotel on Park Lane, a year ago. He told her he was a "political journalist."

Friends and family in Murray's hometown, outside Dublin, said today that he had asked her to marry him and that they were to travel to Israel for the wedding ceremony and a honeymoon. Telling her he would take a later flight to Tel Aviv, he dropped her at the airport yesterday morning.

Police described the bomb, discovered under a false bottom in a piece of baggage she was carrying, as "viable" and timed to go off while the plane was in the air. They said that all aboard, including Murray, who is believed to be six months pregnant with Hindawi's child, would have been killed.

The attempted bombing was one of a series of terrorist acts involving Britain and British citizens since the attack early Tuesday against Libya by U.S. aircraft, some of which were launched from British bases.

Referring to another episode, Foreign Secretary Geoffrey Howe said today there was "firm evidence" that two Britons found dead yesterday in Lebanon had been kidnaped three weeks ago with "Libyan involvement." After their abduction on March 28, Howe said, Britain "had good reason to believe they were in Libyan hands." He provided no details.

[In Washington, diplomatic sources familiar with the Lebanon situation said the two slain Britons were known to have been kidnaped by a terrorist group employed by Libya. However, the sources added, the Lebanese government believes that they killed their captives on their own initiative as a sign of loyalty to Qaddafi rather than in response to a specific order from Libya.]

A third man also found dead in Lebanon yesterday and originally identified as another missing Briton, Alec Collett, was identified today as an American who had been held hostage for more than a year.

Meanwhile, the mother of another Briton who was abducted in Beirut yesterday, television cameraman John McCarthy, today pleaded with his captors to release him. Echoing widespread criticism here of the U.S. air strike against Libya, she said that "he, and we as a family, do sympathize with what has happened, and the British people in general are opposed" to it.

The search for Hindawi began at 9:15 a.m. yesterday, when a concealed explosive was found in Murray's baggage before she boarded El Al flight 016 to Tel Aviv. Police said that they believe Murray, 32, had been "duped" by Hindawi, who had dropped her off at the airport promising to meet her later in Israel, where she believed they would be married.

In other developments, government officials here today confirmed they have contingency plans for the evacuation of as many as 5,000 British citizens in Libya and 1,000 in Lebanon.

Prime Minister Margaret Thatcher said today that she had "agonized" over the decision to allow the use of British bases for the attack on Libya. "Yes, we did agonize," Thatcher said in remarks to the North London Branch of the Conservative Friends of Israel, in her parliamentary constituency of Finchley. "Yes, we did ask whether all the targets" of the air attack "were connected with terrorism.

"It would have been easier to run away, but we did not -- and we would take the same decision again," she said.

"The United States, which has been so generous in defending liberty and freedom, is entitled to ask for help," Thatcher added.