As a result, the official said, work on the text could take several days, and it was unclear whether it would take the form of a resolution or a less forceful ''statement'' issued by the President of the Council.

Mr. Velayati, while denouncing the downing of plane as a ''criminal act,'' an ''atrocity'' and a ''massacre,'' acknowledged in his hour-and-a-half address that the Iranians ''do not think the present session is ready to deal objectively with these blatant acts of aggression by the United States.''

Mr. Velayati, dressed in a light gray suit with a high-collared white shirt and no tie, also alluded to some of the reasons he expected the Security Council to be cool to Teheran's entreaties. Iran has essentially boycotted the Council since it called for a cease-fire in the Iran-Iraq war in 1980, and it has ignored the Council's resolution last year for an immediate end to the war.

In addition, as Mr. Velayati said today, Teheran has frequently denounced the Security Council as biased against it. These actions, along with Iran's attacks on unarmed merchant shipping in the Persian Gulf, have not won it many friends. 'Could Not Have Been a Mistake'

Nevertheless, Mr. Velayati sought to show that the shooting down of the airliner by the Vincennes ''could not have been a mistake.'' The plane was a regularly scheduled civilian flight, Mr. Velayati said, on a well-charted international airway; it was flying at a normal speed for civilian planes and was at the proper altitude, 12,000 feet, and climbing, according to the pilot's last report. The Vincennes had reported its radar showed the plane was flying at only 9,000 feet and was descending, in what seemed to be a prelude to an attack.

Vice President Bush, in answering the Iranian, offered no new information on the incident. Instead he reiterated the Reagan Administration's view that the trouble began when Iranian gunboats fired on an American helicopter from the Vincennes and that the Iranian authorities should have diverted the plane from crossing virtually over the American cruiser. ''They allowed a civilian aircraft loaded with passengers to proceed on a path over a warship engaged in battle,'' Mr. Bush said. ''That was irresponsible and a tragic error.''

Mr. Bush spoke from a prepared text and did not react to Mr. Velayati's reading of the transcript, which made it sound as if the plane was completely unaware of the battle scene below it while the cruiser was equally unaware the plane was openly transmitting normal civilian radio talk. Comment on Hostages Added