Members of a highly secret security unit under the control of Prime Minister Robert Mugabe's office have been linked to some of the dozens of abductions of opposition party supporters in rural Zimbabwe during the first four months of this year, according to human rights advocates and western diplomatic sources here.

Eleven of those kidnaped were discovered three weeks ago in a government jail where they had been held secretly for more than a month, allegedly by agents of the Central Intelligence Organization, a quasi-military unit responsible for investigating security and intelligence matters, according to Michael Auret, chairman of the Catholic Commission for Justice and Peace.

He said lawyers for the commission, a church-supported human rights group here, are seeking the whereabouts of 13 others also believed held by the CIO.

The 11 never were charged formally with a crime nor issued an official detention order, in apparent violation of Zimbabwean law, according to Auret, who also said they were beaten while in custody. He said they were released May 24 after being turned over to the criminal investigation department of the national police force.

The discovery that the 11 were in custody is the first indication that at least some of the politically connected abductions have been carried out by government agents. It also is the first indication of the fate of any of the kidnap victims, many of whom were feared to have been killed.

An official government spokesman, Director of Information John Tsimba, said the government would have no comment on the incident because it was considered a security matter.

In the past, government spokesmen, including Mugabe, have denied any role in the disappearances and have charged, in turn, that 25 Mugabe supporters have been killed and 26 raped by politically motivated bandits since the beginning of the year.

The Central Intelligence Organization, first established under the white-minority government of former prime minister Ian Smith, answers directly to Mugabe through his minister of state for security affairs, Emmerson Munangagwa, who serves in the prime minister's office.

The 11 were abducted from their homes in rural Silobela in central Zimbabwe by unidentified men around April 4, according to Auret. He said the 13 others still being sought were taken from the same area about one month earlier.

Auret's account, given in an interview last week, was confirmed by western diplomatic sources who said that they had independent knowledge of the two incidents.

The Silobela area, which is west of the central city of Gweru, has been the scene of repeated violence between Mugabe's supporters and those of opposition leader Joshua Nkomo.

Plainclothes CIO agents have operated extensively in the area during the past six months following the upsurge of political violence there. The agents, who routinely do not identify themselves, began a crackdown on Nkomo sympathizers in March after armed dissidents allegedly were sighted in the area, according to Auret. Witnesses said the 24 men were taken by unknown men during several late-night raids and hauled away in an unmarked vehicle.

Auret said the commission's lawyer had ascertained that the men had been picked up and held on authority of the CIO. In the case of the 11 seized in April, he said, it was only after the men were taken to an official holding camp in Silobela that they realized they had been taken by government agents. Their families never were informed that the men had been taken into custody, Auret said, nor were they allowed to contact lawyers.

Zimbabwean law requires that those being held be informed within seven days of the reason for their detention and that within 30 days they either be charged in court or be detained by written order from the minister of home affairs, the Cabinet official in charge of police. Detainees also are entitled to see an attorney. Auret said most of the 11 men were held for more than 30 days with no charge or detention order.

Auret said written orders were issued on May 10 only after a lawyer for the Peace and Justice Commission discovered that the men were being held in a government prison in the central town of Kwe Kwe.

Another man, allegedly abducted from the Tsholotsho area in southwestern Zimbabwe several months earlier, also was discovered at the Kwe Kwe prison and has since been released, Auret said.

None of the other abduction victims has been found yet. "We simply have no idea at all where they possibly might have been put," Auret said.

Auret said the men were told that they were being held because they had aided the dissidents, who the government alleges are working clandestinely for Nkomo.

All were Ndebele, the minority ethnic group that is Nkomo's base of support, and were known as Nkomo supporters despite the fact they all possessed membership cards of Mugabe's ruling party, the Zimbabwe African National Union. Many of Nkomo's followers have purchased the cards during the past three years as a form of protection against assaults by Mugabe supporters or the Army.

Nkomo repeatedly has denied any link to dissidents and earlier this year submitted a list to Parliament of 53 members of his opposition political party who he said had been abducted by unknown men during the first three months of this year. Most were taken from their homes in southwestern Matabeleland, Nkomo's political stronghold, by men dressed in plainclothes and using unmarked vehicles, according to witnesses.

While there are no definitive figures, diplomatic and other sources believe at least 75 to 100 persons have "disappeared."

Informed sources say several government soldiers were killed last week in an exchange of gunfire with dissidents and soldiers in the Esgodini area south of Bulawayo, an incident that has not been confirmed officially.

The nonaligned, socialist-oriented Mugabe government cited its statistics of murders and rapes earlier this month in winning parliamentary approval for a six-month extension of emergency powers that have been in effect since 1960. They empower security officials to detain without charge or trial any person believed to be a threat to public safety.

Mugabe and Nkomo were rival guerrilla movement leaders who formed a governing coalition in 1980 after the advent of black majority rule here. Their alliance dissolved in 1982, when Mugabe, whose party holds a majority of seats in Parliament, accused Nkomo of plotting to overthrow the government. Since that time, the government's security forces have engaged in a series of crackdowns on Nkomo's followers in the midlands area of Matabeleland where Silobela is located.