YANGON, MYANMAR -- Government authorities, preparing for the visit of a U.N. human rights investigator, have begun to whitewash prison walls, improve inmates' diets and coach political prisoners on how to answer questions, according to knowledgeable Burmese and foreign sources here.

Sadako Ogata, a Japanese professor named recently as a special rapporteur for the United Nations on Myanmar, formerly Burma, is expected to arrive in the next few days to investigate a human rights situation that diplomats describe as among the worst in Asia.

Although the country's ruling military junta appears to be taking steps to control the rapporteur's visit tightly, students and opposition politicians are looking forward to it as an opportunity to expose what they regard as severe human rights violations and stifling political repression.

International human rights organizations have charged the martial-law regime with a record of abuses that includes the massacre of at least 2,000 demonstrators in August and September 1988, thousands of arbitrary arrests, the routine torture of political prisoners, the forced relocation of up to 500,000 persons and the use of dragooned civilians as human minesweepers and military porters in counterinsurgency operations.

Among the latest actions by the ruling State Law and Order Restoration Council were the arrests on unspecified charges last week of at least a dozen leaders of the opposition National League for Democracy. The league decisively won elections last May for a new People's Assembly, but has been prevented by the junta from assuming power.

In September, about 90 political prisoners at Insein jail on the outskirts of the capital staged a hunger strike to mark the second anniversary of the junta's Sept. 18, 1988, takeover and the subsequent massacre of protesters.

According to Burmese and foreign sources, the army suppressed the hunger strike Sept. 25, beating at least three protesters to death with clubs wrapped with barbed wire and injuring at least 40 others. Some accounts put the death toll in the incident as high as six. Scores of inmates reportedly were transferred to other prisons.

In Bangkok, the Myanmar Embassy denied in an Oct. 16 statement that any prisoners were killed in the incident, but said three suffered "slight injuries." The statement said that "in one instance, internationally recognized batons were unavoidably used, but were not wrapped by barbed wires."

According to Burmese dissidents and diplomatic sources, authorities last week began whitewashing the walls of Insein jail, giving prisoners new blankets and mosquito nets and providing better food -- including meat four times a week -- in preparation for Ogata's visit. An informed diplomat said the inmates also were being instructed what to tell Ogata, a professor at Tokyo's Sophia University who formerly served as Japan's first representative to the U.N. human rights commission.

One resident of Yangon, the capital formerly known as Rangoon, described the junta's actions as an effort to create a "Potemkin prison."

A student group has indicated that it might try to stage a demonstration during Ogata's visit, but the military appears prepared to snuff out any such effort instantly. Some student leaders reportedly were arrested recently for trying to write a statement to present to the U.N. representative. Nevertheless, Burmese have been besieging the Japanese Embassy here with requests to meet her or pass on reports of human rights abuses.

In a sign of the junta's intolerance of public protests, high school students who tried to drop flowers one by one in front of the U.S. Embassy in August were rounded up by Burmese intelligence agents within minutes and hauled away, witnesses said. The students' gesture was meant to commemorate the deaths of classmates who were gunned down there during massacres in August 1988.

According to the U.S. human rights group Asia Watch, the army killed "at least 1,000" pro-democracy demonstrators on Aug. 8, 1988, by opening fire on unarmed crowds in the capital. "Eyewitnesses also described the bayoneting of demonstrators, including children, by army troops," the group said in a May 1990 report, adding that "soldiers were said to have removed bodies and cremated them in secret." It said an estimated 1,000 to 3,000 other people were killed in September 1988 when troops fired into crowds of demonstrators and houses to crush protests against the junta's takeover.

The government of Myanmar has claimed that fewer than 550 persons were killed in August and September 1988, including only one woman.

Asia Watch and Amnesty International also have reported widespread torture of prisoners in Myanmar by such means as beatings, electric shocks to the genitals, cigarette burns, near drownings and the rolling of iron bars up and down the victim's shins until the skin peels off. According to Amnesty International, at least six state security units practice torture, most notably the military intelligence agency, the Directorate of Defense Services Intelligence.

Political sources in Yangon estimate that as many as 3,500 people have been arrested for political reasons this year, most of them since the May elections, and that 1,500 to 2,000 political prisoners are still being held. Other estimates are much higher.

The most prominent political prisoner is Aung San Suu Kyi, the 45-year-old leader of the National League for Democracy, who was placed under house arrest in July 1989 and prevented from running in the May elections. Although her detention was supposed to be for one year, the junta has shown no sign that it intends to release her. Soldiers armed with automatic weapons continue to stand guard around her house on University Avenue 24 hours a day.