The FBI has thwarted a plot by Indian Sikhs to assassinate Indian Prime Minister Rajiv Gandhi and another Indian official during visits to the United States and to bomb a nuclear power plant and other strategic sites in India, FBI Director William H. Webster said yesterday.

Affidavits filed in New York said the plot -- intended to "cause the revolutionary overthrow" of the Indian government -- was discovered by an operative identified only as "A" and as a winner of the Congressional Medal of Honor for service as a Navy commando in Vietnam.

Gandhi, son of Indian prime minister Indira Gandhi, who was killed by Sikh bodyguards last year, is scheduled to make a state visit to the United States June 12 to 15, with stops in Washington and Houston.

The other Indian official, Bhajan Lal, chief minister or governor of the state of Haryana, was in New Orleans for medical treatment at the Louisiana State University eye center but reportedly returned to India Friday.

In a statement, Hardam Singh Azad, chairman of Sikh Association of America, appealed last night to the Justice Department and FBI to investigate the background and connections of individuals behind what he called "these despicable activities."

Three persons have been charged in New York with plotting to assassinate Gandhi. Two of them, identified as Lal Singh and Ammand Singh, remain at large.

The third, Gurpartap Singh Birk, who uses the alias John Singh and lives in Brooklyn, is being held in New Orleans on charges of plotting to assassinate both Gandhi and Lal.

Four other persons have been charged and are in custody in Louisiana in connection with the plot to kill Lal.

"Once again," Webster said in a statement, "the FBI has been successful in detecting and interdicting a terrorist action before it was carried out."

Webster added that the Federal Bureau of Investigation has taken "appropriate steps" to make sure that terrorists "do not succeed in getting even a beachhead such as they have enjoyed for years in other parts of the world."

According to the affidavits, "A" uncovered the plot in January at a meeting with Birk and Lal Singh, in a New York hotel room. "A" told them he had become an expert in the use of explosives and automatic weapons while in Vietnam, according to the affidavit.

Birk and Singh then told "A" they were planning to assassinate Indian political figures, including Gandhi, and to overthrow the Indian government, according to the affidavit.

The two said they also intended to create a loss of confidence in the Indian government by bombing strategic locations in India, including a nuclear power plant, bridges, hotels and government buildings.

The affidavit said they asked "A" to train a group of men in the United States in the use of explosives and weapons so they could carry out their plan in India.

They also asked him for small arms, machine guns and explosives -- enough to blow up a large bridge and a 36-story building, according to the affidavit. The source said he could supply the material.

The affidavit said Birk and Singh discussed the planned Gandhi assassination at another meeting, and "A" said he could suggest "three possible plans." Birk and Singh then selected one, it said.

The FBI has videotapes of several of the meetings, according to the affidavit.

The FBI obtained evidence of the other alleged plot -- to kill Lal -- during a telephone conversation between Birk and an FBI agent.

Birk and three others were arrested near Lal's hotel room on May 4. A fifth man, a New Orleans cab driver, was arrested Sunday.

A federal grand jury in New Orleans returned an indictment on Thursday charging Birk, Sukhvinder Singh, Virinder Singh and Jasbir Sandhu, all of New York, and the cab driver, Jatinder Singh Ahluwalia, of conspiring to kill Lal