A leading opponent of martial law who has been hiding from police for six weeks has escaped the Philippines and plans to go to the United States, dissident sources said yesterday.

Charito Planas, an attorney and outspoken critic of Philippine President Ferdinand Marcos, reportedly left about three days ago, but her friends would not say how she left. Dissident sources here said the usually heavy set Planas had altered her appearance by going on a crash diet after she went into hiding following an April 7 police raid on her home.

Vice President Walter Mondale had indicated to anti-Marcos leaders here May 3 that if Planas eluded police and reached the United States she would be allowed to stay. As a member of a slate of anti-Marcos candidates for new interim National Assembly last month, she received nearly one third of about 3.1 million votes cast in Manila. On that same day, police confiscated what they said was an illegal automatic rifle and "subversive literature" found in her house and she dropped out of sight.

(Mondale and key aides were in New York yesterday for his address to the United Nations, and could not be reached for comment.)

Former Philippine Senator Rau S. Manglapus, reached by telephone at his home in McLean, Va. said yesterday that Planas had left the Philippines and would soon arrive in the United States. Manglapus declined to say where Planas was now. Like dissidents here, he declined for the time being to give any more details about her escape.

Manglapus said he expected Planas would soon join him and other Philippine exiles who are lobbying the U.S. Congress to cut aid to the Marcos government.

Planas was one of the most vehement and energetic critic of Marcos during the assembly election campaign. Allowing her to remain in the U.S. would add one more irritant to U.S. Philippine relations that have become strained over the human rights issure and the presence of two hug U.S. military bases in the former American colony.

Manglapus and other critics recently persuaded the U.S. House of Representatives to cut about $5 million from one bill for military aid to Manila. "I think that perhaps the Congress itself might want to have (Planas) come and say something about what is happening," Manglapus said.

In escaping the country, Planas both avoided arrest and eluded a ban on overseas travel that has been ordered for about 460 Marcos opponents, including apparently every assembly candidate who ran against the victorious Marcos-backed slate in Manila last month. The country's leading Roman Catholic clergyman, Jaime Cardinal Sin, was briefly delayed at the airport earlier this month because his name was on the travel blacklist.

Former Philippine President Diosdado Macapagal, Marcos' immediate predecessor, said yesterday that he has so far been unsuccessful in securing permission from the government to travel to Iran next month for a conference of university leaders.

Planas was jailed at the beginning of martial law in 1972, but was later released and allowed to campaign vigorously against Marcos early this year. Marcos charged after the police raid on her house that she had been sheltering leaders of the communist New People's army.

The effort to arrest Planas was part of a general crackdown on dissidents that followed Marcos' sweeping victory in the assembly election. Four other anti-Marcos candidates were jailed for leading a peaceful, but illegal, protest march after the election. Another candidate, slum organizer Trinidad Hererra,was charged with subversion but has apparently not yet been found by police. The leader of the anti-Marcos slate, former senator Benigno Aqunino, has been in jail since 1972 but was allowed to campaign from his cell.