The Uganda crisis took another strange turn yesterday, with the East African country reviving an earlier theme in its war of nerves by claiming that 2,600 American, British and Israeli mercenaries were advancing through Kenya to invade Uganda.

The claim was instantly rejected by Washington, London and Nairobi. There was no immediate comment from Israel, which raided Uganda's Entebbe Airport last July to rescue about a hundred hostages held by Palestinian guerrillas.

Kenya's denial seemed to predict, however, that a revolt could be in the offing inside Uganda.

"It shouldn't require mercenaries to put things right in Uganda," a Kenyan spokesman was quoted by the official nes agency as saying. "Kenya is confident that Ugandans themselves in the not too distant future will be able to have a say as to how their country should be governed."

Uganda Radio, monitored in Kenya, said that information that 2,600 mercenaries were marching toward Kampala came from a letter an unidentified "group of Kenyan citizens" had sent to the Ugandan Defense CounciL.

The Ugandan broadcast also charged that Kenya is becoming "a base for imperialists." In addition to the mercenaries, it claimed that an American air squadron is stationed 80 miles north of Nairobi and a U.S. destroyer is in Mombasa, a Kenyan port.

Last week, Amin claimed that U.S., British and Israeli paratroopers were planning to support an invasion and coup attempt by the exiled former president of Uganda, Milton Obote, who is living in Tanzania.

Amin said in another broadcast yesterday that there were about a hundred Americans in Uganda, some of them illegally.

The U.S. State Department has estimated that there are about 240 U.S. citizens in Uganda, most of them missionaries and teachers.

Amin repeated assurances that all Americans are free to leave but said those in the country illegally will be required to explain their presence to the Ministry of Internal Affairs.

He said that such illegal immigrants "may be criminals and therefore dangerous to society."

The Americans, now freed from travel restrictions, do not appear in any hurry to leave. An Air France jet that flew from Entebbe Airport to Nairobi today picked up 29 passengers in Uganda, but none of them was American.

An American tourist arriving in Nairobi from Uganda said he had been held without charge by military police who questioned him repeatedly and took him on unexplained rides. He said that on one occasion he had been awakened by "drunken soldiers wanting to beat up some prisoners."

"I was pretty sure I would be bumped off," said Brian Schwartz, 24, a New York City lawyer. Schwartz said Ugandan military police had picked him up Saturday, whil he was having a beer in a market.

Officials of the civil police indicated to him that any European in Kampala was likely to be picked up and that the police had no control over the military authorities and could not guarantee the safety of any Westerner in Kampala, Schwartz said.

Meanwhile, Radio Uganda said that Amin had given a final briefing to his country's charge d'affaires to the United States, Mahmud Musa, who was leaving for Washington last night. It did not give any details.

In another broadcast, the Ugandan leader was quoted as saying that he has 31 children, "some of whose mothers may be Americans." Amin said that the mothers come from various African tribes and "some of them belong to different nationalities." He also said he expects to have more children.