Frequent torture cases, disappearances and killings have continued in the Philippines following the lifting of martial law more than a year and a half ago, Amnesty International charged in a report on human rights conditions there.

A summary of the report, released on the eve of Philippines' President Ferdinand Marcos' first visit to the United States in 16 years, put particular emphasis on the activities of the country's armed forces and paramilitary groups which, Amnesty charged, operate with tacit government approval.

Marcos is to arrive in Washington today, and is to meet with President Reagan Thursday. Five Democratic senators expressed fears in a letter to Reagan Monday that the Marcos visit will be misinterpreted as a sign that the United States "condones . . . continuing violations of basic human rights" in the Philippines.

"The security of the United States does not require our support for the repression of the Filipino people," the letter said.

It was signed by Sens. Claiborne Pell of Rhode Island, Alan Cranston of California, Edward M. Kennedy and Paul E. Tsongas of Massachusetts and Daniel Patrick Moynihan of New York. Pell, Cranston and Tsongas are members of the Senate Foreign Relations Committee. Marcos is to meet with the committee Friday.

The latest report by Amnesty International noted some improvement in the number of people detained for political reasons compared with an earlier report of a 1975 mission the group sent to the Philippines, giving the figure 1,000, down from 6,000 in 1975.

The new study noted, however, that, "Reports in the same period indicated more people were becoming victims of human rights violations of unusual brutality, including 'disappearance' and extrajudicial execution."

Amnesty said it had documented 230 cases of disappearances between 1975 and 1980. Disappearances, people being taken into custody without official responsibility being admitted, have been common in Latin America in recent years, but had not been part of the Philippines political scene until recently.

The Amnesty summary attributed many of the rights abuses to a three-fold expansion of the country's armed forces over the past several years as the Marcos government has sought to counter an ongoing Moslem insurgency in the south of the country and the growth of the anti-government New People's Army.

The Amnesty report charged that rights abuses "are common, particularly in areas where the New People's Army is reported to have established a presence."

" . . . In a high proportion of cases killing occurred after interrogation and torture or after the victim had been taken to a place of detention . . . . No evidence was found in any of the cases investigated that the victims were killed in encounters with military or police personnel as the authorities have sometimes alleged," Amnesty said.

U.S. officials have said several times in recent weeks that the Marcos government is aware that it has problems with some segments of the armed forces but has been unable to bring the situation under control because of the rapid buildup of the military in recent years.