Scotland Yard's antiterrorist squad is searching for an Iraqi man who is believed to have been the organizer of the siege of the Iranian Embassy here that was ended by a British commando raid last week.

The suspect, who identified himself to a London rental agent as Sami Muhammed Ali and gave a Baghdad address, had leased an apartment here for the six ethnic Arab Iranian gunmen who held the embassy and more than 20 hostages for more than five days.

Five of the gunmen were killed in last Monday's successful commando raid on the embassy. Only one survived by infiltrating the fleeing hostages before he was arrested. He has apparently told police about the apartment used by the gunmen to plan the embassy seizure and about the seventh man now being sought by Scotland Yard.

The suspect is described as an older, balding man who disappeared when the siege of the embassy began, leaving behind an address and telephone number in Baghdad. People answering that number have refused to talk with reporters calling from London.

The six gunmen were in their twenties and told police negotiators they were Arab militants from the oil-producing Khuzestan Province in south-western Iran near the Iraqi border. Iraq has openly aided militants among the large Arab minority there who have clashed with Iranian troops and sabotaged oil pipelines.

All seven men are believed to have arrived in London a month before the Iranian Embassy was seized. It is thought that they were first in a boarding house and then in a well-appointed short-lease apartment. People in the neighborhood say they spent a great deal of time drinking in a local pub and spent large amounts of money on new clothes and other items that police found packed into suitcases when they raided the apartment following the siege.

Police also reportedly used sophisticated eavesdropping devices to monitor conversations among the gunmen inside the embassy. The police reportedly learned that the seventh man had promised the gunmen that they would be able to negotiate successfully with the British authorities for publicity for their cause and safe passage out of the country.

When the British government steadfastly refused to allow them to leave the country, the gunmen executed at least one hostage and said they would kill more until their demand was met. That prompted British officials to order the commando raid, which rescued 19 hostages.