A bizarre kidnaping drama came to a close here this morning when a prominent Arab businessman, who was abducted from his London home 11 days ago, reappeared chained to a bed frame at the front door of a suburban house.

Sadiq Tajir, 45, apparently had been abandoned by his captors and allowed to escape after his brother, the ambassador to Britain from the United Arab Emirates and self- described "richest man in the world," paid $3 million for his release.

The ransom was the highest ever paid in a kidnaping in Britain. It was handed over, in the form of a bank draft signed by Ambassador Mohammed Mahdi Tajir, to an intermediary identified by police today as "Mr. X."

The kidnapers, described as four Arab men, have not been found, and police said it was not known if there was any political motive involved.

According to Scotland Yard, Ambassador Tajir initially refused to pay the ransom for his brother, a decision with which they concurred. He eventually was persuaded, they said, by other family members.

The ambassador is former chief of customs in Dubai, and his personal wealth is estimated at about $3 billion. In a recent television interview, he said reports that he was the richest man in the world were accurate. His homes in this country include a number of luxury apartments in London, castles in Scotland and Kent and Windlesham Moor, a former home of Queen Elizabeth and Prince Philip near Ascot.

Regarded as a somewhat controversial diplomat, he was rebuked by the British government for failing to take a seat reserved for him at the 1979 opening of Parliament, an occasion of high ceremony attended by the queen.

He has served two tours as ambassador here. He resigned briefly in 1982, after 10 years, reportedly to avoid becoming dean of the diplomatic corps, a position requiring extensive social and bureaucratic commitments. He returned in 1983, when the queen reciprocated his earlier apparent snub by inviting him to present his credentials on a day when she was out of town.

"Mr. X" had made the initial ransom calls from New York City, beginning on Jan. 7, the day after the kidnaping. At that time, he demanded about $72 million. He then traveled to Europe, where a nephew of the ambassador handed over the bank draft, and finally to Beirut, where the draft was cashed on Wednesday.

Authorities said they knew the current whereabouts of Mr. X but declined to disclose them. They refused to say whether he was under arrest, although officials said they hoped to "interview" him within the next several days. They said that most of his movements -- spanning three continents throughout the negotiations -- had been "monitored" with the assistance of law enforcement agencies in other countries.

Throughout the negotiations, police kept local journalists informed of their progress and promised a full briefing when the kidnaping was over. Under a 10-year-old agreement with the British media, Scotland Yard ordered that nothing be broadcast or printed as long as Tajir was being held.

Ambassador Tajir, it was revealed today, contacted Scotland Yard as soon as he received the first ransom call the day after his brother disappeared.

In the next 10 days, the ambassador received a series of telephone calls from Mr. X. All the conversations were conducted in Arabic with Scotland Yard officials listening in after the initial contact was made.

On Jan. 8, the ambassador received the first of what were to be four letters from his brother, all of them mailed in London. Mailed the previous day, the letter said: "I am kidnaped. My life is in danger. Please do not contact any authority or western official or any others."

It was arranged for the ambassador's nephew to carry a bank draft for $3 million, the negotiated ransom, to Geneva. It was there that police shadowing the nephew were able to place Mr. X under surveillance. Mr. X subsequently flew to Zurich, then to Rome and finally to Beirut.

Throughout these events, the ambassador demanded proof that his brother was alive before authorizing clearance of the bank draft. Last Sunday, he received a call telling him to pick up a package left behind a park bench in central London. There, an envelope was found, containing a Polaroid photograph of Sadiq with a Sunday newspaper.

Mr. X telephoned several times from Beirut and, on Wednesday, law enforcement authorities watched him cash the draft at a Beirut bank. At 10 o'clock last night, Mr. X called to say that the money had been given to his principals and that the victim would be released soon.

This morning, police said, Sadiq, who told them later that he had been chained hand and foot to a bed, blindfolded and drugged throughout the ordeal, awoke to silence. Removing the blindfold, he found a note, written in Arabic, beside him. It read: "You are going to be released. Stay very quiet. Don't shout for help. When the time comes, you will be released. If you shout for help, you will be dead."

Having remained quiet in accordance with the instructions for some time, Sadiq at 10:30 a.m. "managed to remove the mattress underneath him and manhandle the bed, to which he had been chained, out of the room and down the stairs, then through the front door," police said.

He found himself in a row house on Harpenden Road in south London. Next door, Nora McNaughton said she heard a commotion and opened her door to find him standing outside, still chained to the bed and dressed in pajamas and socks.