The whereabouts of Ugandan President Idi Amin were a mystery to the outside world today following reports here that he was the target of a weekend assassination attempt.

A Nairobi newspaper that has long been hostile to Amin said that Amin had disappeared, according >TO> sources, after gunmen tried to kill him Saturday in a town near Entebbe. There were also reports here of troop movements, roadblocks and unusual security activities in Uganda.

Ugandan officials reached by telephone denied the assassination attempt report but shed no light on Amin's whereabouts. "We don't know where he is and nobody has any explanation," an unidentified man who answered the telephone at Amin's home in Kampala told a reporter.

Observers here and elsewhere outside Uganda reacted to the fragmentary reports with skepticism and caution, noting that Amin's government carried out an elaborate hoax two weeks ago, announcing that the unpredictable leader was en route to the British Commonwealth conference in London, where he was not wanted. Uganda later admitted Amin had not left Africa.

[U.S. officials said they had no clear information about Amin. "What do you hear about Idi Amin?" a newsman asked Zbigniew Brzezinski, head of the National Security Council. Silently, Brezezinski made the sign of the cross.]

Some reports circulating here among exiles with contacts inside Uganda said Amin had been wounded in the attack and flown to Libya for medical treatment. There was no mention of Amin, however, from Libya, which has become his closest ally.

The last official report on Amin's whereabouts came Saturday when Radio Uganda reported that the was presented with a giant fish caught in Lake Victoria by his British-born aide, Bob Astles.

Amin was last heard live on Radio Uganda on Thursday when the annual budget speech was read at a meeting in Kampala.

The government-controlled radio said early today that Amin might attend a Refugee Day celebration today in northwestern Uganda. Later broadcasts had no mention of the celebration or of Amin.

The Daily Nation, the Nairobi paper that reported there had been an assassination attempt, has been a longtime critic of Amin and has frequently carried reports of atrocities by his regime. Its dispatches have not always proved completely accurate.

The paper said Amin's disappearance was confirmed by Ugandan Vice President Mustafa Adrisi, whom it quoted as saying, "If you find him, please contact Uganda immediately."

Other reports here, however, said the vice president was also a target of the attack.

There were reports, meanwhile, from sources in Tanzania close to former Ugandan President Milton Obote, whom Amin overthrew in 1971, that Amin is being held prisoner by hostile factions of the Ugandan army.

Several sources here have received reports of shootings in Entebbe on Saturday morning and of an extra heavy guard on duty outside government buildings in Kampala.

Some Ugandan refugees here suggest that the alleged assassination attempt may have been another hoax by Amin in an effort to get covert opposition to come into the open.

The Ugandan strongman, who is secretive about his moves for security reasons, has reportedly survived numerous assassination attemps since taking power in a military coup in January 1971. Last February, he ordered the arrest of several key figures, including Anglican Archbishop Janani Luwum, for allegedly plotting a coup, a charge the archbishop denied.

A day after the archbishop's arrest, Radio Uganda announced Luwum and two government ministers arrested with him had been killed in a traffic accident while attempting to escape. Religious groups outside the country charged the three had been murdered as part of a purge against Uganda's two large Christian tribes.

There have been numerous reports of atrocities in Uganda during Amin's rule, although independent verification has been difficult because of Uganda's strict control of journalists. The latest allegation came earlier this month when former Health Minister Henry Kyemba, who fled to Britain, charged Amin's troops and police had killed some 100,000 Ugandans during his six-year rule.

"Even Amin does not know how many people he has ordered executed," Kyemba said. "The country is littered with bodies."