Although the era of the Khmer Rouge mass murders has ended in Cambodia, systematic torture and arbitrary arrest of thousands of political prisoners continues under the current Vietnamese-backed government, according to a 250-page report released yesterday by a Washington-based human rights group.

The report by the private, nonpartisan Lawyers Committee for Human Rights said torture included beatings, electric shocks and lye powder thrown in victims' faces. The torture is often supervised by Vietnamese "experts," it said.

It is estimated that there are thousands of political prisoners, according to the report, which accused the Cambodian government of trying to "stamp out opposition -- both real and imagined."

Based on fact-finding missions last October and in February, the report is the most extensive study of human-rights conditions in Cambodia since Vietnam invaded that country in late 1978, ousted then-Prime Minister Pol Pot and, in January 1979, installed the regime of Heng Samrin, the State Department said yesterday in a lengthy statement.

The report is also critical of human-rights abuses in areas under the control of two of three rebel groups, the Communist Khmer Rouge and the major noncommunist group led by former prime minister Son Sann.

The House and Senate recently voted to provide overt assistance to the noncommunist resistance -- Son Sann's group and another headed by Cambodia's former head of state, Prince Norodom Sihanouk -- for the first time since 1975.

Floyd Abrams, a prominent New York attorney, headed the three-member delegation. The report is based on interviews with more than 150 persons, including refugees on both sides of the Thai-Cambodian border, relief officials, U.S. officials and journalists.

A State Department spokesman said the committee's findings in general were consistent with reports received of "widespread use of torture and other forms of mistreatment by Vietnam and its client regime in Cambodia to extract alleged 'confessions' from persons suspected of opposing the regime installed by Vietnam."

The Vietnamese mission to the United Nations had no immediate comment. In the past, Vietnam has rejected such charges. Cambodia has no diplomatic representation in the United States.

The report's authors noted that although the current violations "pale in comparison to the mass murders" of the Pol Pot regime, they "rival those of other nations whose abuses have provoked outcries from the world community."

A typical example described was the case of a man arrested in 1980 while bringing rice from the Thai-Cambodian border to his nearby village. Accused of cooperating with the noncommunists, he was tortured with electrodes connected to his hands and arms, and beaten by a Vietnamese interrogator. After 12 days, he "confessed" and was jailed for three years and eight months.

In areas under Khmer Rouge control, the report described a brutal system of social control, where people are imprisoned for marrying or taking photographs without permission. Despite the Khmer Rouge leadership's efforts to project a "more moderate image, draconian measures were used to enforce even the image of reform: persons . . . could be imprisoned for making such comments as 'Things are being done the way Pol Pot used to do.' "

The State Department said the report's findings in those areas were "valuable because few independent observers have gained access to Khmer Rouge areas."

The spokesman said the report's findings "support our belief that the Khmer Rouge remains a totalitarian organization determined to dominate completely the lives of persons under its control."

In areas under control of the major noncommunist group, the Khmer People's National Liberation Front (KPNLF), the committee found, despite efforts by the group's leadership to enforce discipline and establish the rule of law, episodes where noncombatants and troops were beaten, detained under harsh conditions and sometimes killed by military or administrative personnel.

The State Department said the report noted the "KPNLF leadership's commitment to respect for basic human rights and the problems in translating that commitment into reality."