An ally of former president Ferdinand Marcos, Arturo Tolentino, proclaimed himself acting president today after taking over a plush Manila hotel with the support of about 400 armed Philippine soldiers, but hours later most of the rebels left in what appeared to be a waning revolt.

The government of Corazon Aquino sent three envoys to the Manila Hotel who negotiated with the rebels for one hour around midnight. Shortly thereafter the soldiers began to leave, and by midday today only about 180 remained in the hotel.

No violence was reported and Aquino's executive secretary, Joker Arroyo, said on the radio that the government is keeping its options open in dealing with the Marcos supporters. "The expected groundswell just fizzled out," he said.

Tolentino, 75, who was Marcos' running mate in the February presidential elections, proclaimed a new government in Marcos' name, until the ousted president could return from exile in Hawaii.

Aquino, who had left the capital yesterday for a visit to the southern Philippine island of Mindanao, returned to Manila's presidential palace this morning and conferred with the military chiefs. She refused to make any statement.

In a news conference in Mindanao, Aquino told reporters yesterday that she was in control and accused Marcos of trying to destabilize her government. She said Tolentino may face sedition charges.

Her two top military leaders, Defense Minister Juan Ponce Enrile, who had stayed in Manila, and Chief of Staff Gen. Fidel Ramos, who had accompanied Aquino on her trip, quickly signaled their loyalty to her.

The takeover of the hotel began when the soldiers in military trucks and jeeps arrived at a downtown park to join an estimated 5,000 Marcos supporters holding their weekly rally. Many of the troops carried machine guns and wireless radios. They said they came either from military bases in Manila or from camps in central Luzon.

Tolentino, who was administered an oath in the hotel driveway by an ousted justice of Marcos' supreme court, immediately named an initial five-man Cabinet that included Enrile as minister of defense and prime minister.

Enrile, who also served as Marcos' defense minister before Aquino came to power, said he turned down Tolentino's offer.

Enrile and Ramos led the civilian revolt against Marcos in February that brought Aquino to power.

In Washington, the Reagan administration said it opposed a new government and reaffirmed its support for Aquino. "The United States strongly supports the government of President Aquino and is against efforts such as these to undermine it," Joe Reap, a State Department spokesman, said. A State Department official who asked not to be identified described the incident as a "tempest in a teapot."

He said there was no hard evidence linking Marcos to Tolentino's move, "but one would have to be pretty naive to think that he didn't have anything to do with it."

In Honolulu, a spokesman for Marcos said yesterday that the former president was monitoring the events closely and would go home to the Philippines as an "elder statesman" if the Aquino government were overthrown. Although Tolentino said he received a letter from Marcos which he said authorized him to act as president, Marcos spokesman Joe Lazo said Marcos had not known about Tolentino's plans in advance, United Press International reported.

However, several sources who were present last night when Tolentino and his top advisers spoke to Marcos by telephone said that the former president had insisted he would not return, The Los Angeles Times reported.

"President Marcos told them to avoid bloodshed at all costs and to accept any reasonable political settlement to get out of the situation," said a Filipino journalist loyal to the former first family who listened to the conversation. "Everyone seemed a little shocked. It was like he was calling the whole thing off and saying, 'Take the first thing they offer you and get the hell out of that hotel,' " the Times report said.

Defense Minister Enrile, after meeting with 20 commanding generals at armed forces headquarters, told reporters yesterday, "We want to solve the problem in a way that will not affect armed forces unity." In a message to the public, he called for calm and said, "There is no reason to panic. This situation has been assessed by us and it is manageable."

There were reports that some of the soldiers participating in the hotel takeover were misled into believing that they were coming to Manila to support Enrile and Ramos.

Except for the supporters gathered outside the hotel -- the building that used to be Gen. Douglas MacArthur's headquarters before and during World War II -- people seemed to be in no hurry yesterday to rally to the Marcos cause. They strolled in nearby Rizal Park, or watched soccer on television as on any other Sunday. The airport was open.

The government reacted by posting antiriot troops and soldiers loyal to Aquino on roads leading to the presidential palace, about three miles from the hotel. Roads were lined with barbed wire, and trucks and fire engines were posted to block approaches to the palace. Reuter reported that a Navy gunboat was anchored in Manila Bay in support of the pro-Marcos forces.

Ramos, while still in Mindanao with Aquino, said, "We have maintained a policy of dialogue with the Marcos loyalists." He added that he and Aquino were in constant touch by radio and telephone with Manila and had appealed to the soldiers' sense of duty to abandon what he called the "illegally installed Tolentino," the domestic Philippine News Agency reported.

Vice President Salvador Laurel was in Spain on an official three-day visit. Speaking to reporters on arrival in Madrid, he said the group of Marcos supporters "is very small and dwindling; they have no chance of success."

Tolentino was joined in the hotel lobby by several military officers ousted from their posts by the Aquino government, including Brig. Gen. Jose Zumel, former head of the Philippine Military Academy.

Several colonels were also seen in the crowd of about 500 milling in the lobby.

Also gathered at the hotel were provincial governors and town mayors fired by Aquino. They said they were throwing their support behind Tolentino.

In a telephone interview with National Public Radio, Tolentino said he did want to have any "real confrontation" with Aquino, but said he hoped she would "listen to the will of the people."

He said he could not stay at the Manila Hotel "for long" because "it's quite an expensive hotel," but added that he would continue to "assert the authority as the legitimate government."