A caption on Page A33 yesterday incorrectly named Indian Prime Minister Rajiv Gandhi's grandfather. He was Jawaharlal Nehru.

Indian authorities today disclosed the arrest of a third Sikh security guard on suspicion of involvement in the assassination of prime minister Indira Gandhi.

According to the United News of India news agency, Jagtar Singh, a police constable assigned to the prime minister's residence, was arrested Oct. 31, the day of the assassination, on a firearms charge and an alleged connection with the murder.

Like Gandhi's two presumed assassins, Jagtar Singh belongs to the Sikh religion, whose more militant followers have been waging a violent campaign for autonomy in India's northwestern Punjab State and have come under a government crackdown.

A police report submitted to a Delhi judge, O.P. Gogne, said Jagtar Singh bunked next to Satwant Singh, one of the presumed assassins, in a police barracks and that a 9-mm "cartridge" was found in his shirt pocket during a search after the murder, the news agency said.

The police constable's lawyer, M.S. Atwal, argued that his arrest for an arms-act violation had nothing to do with the assassination and that he was not even on duty that day. Atwal said the cartridge had been "planted" on his client because he is a Sikh.

However, Judge Gogne refused bail for Jagtar Singh today after a public prosecutor maintained that his arrest was connected to the assassination and that he was likely to abscond if released.

The prosecutor, V.A. Gupta, submitted that Jagtar Singh was involved "in the criminal conspiracy hatched for the assassination of Mrs. Gandhi and his release would seriously hamper the investigation of the sensitive case," the news agency reported.

Judge Gogne also said that his reading of the police report showed that Jagtar Singh had been arrested in connection with the assassination, the agency said.

The police report appeared to raise the possibility that other unnamed conspirators may be at large. The news agency quoted it as saying "the coaccused conspirators have yet to be arrested."

Satwant Singh, a 21-year-old security guard who is accused of shooting Gandhi with a Sten gun, is recovering in a hospital from bullet wounds he suffered in the attack. The other presumed assassin, Beant Singh, 36, died when he was shot by security guards. Most Sikhs carry the surname Singh, meaning "lion."

In a separate development today, the new prime minister, Rajiv Gandhi, expressed "grave concern" over what he said was an escalating armed conflict in Central America.

Speaking as chairman of the Nonaligned Movement, a post he inherited from his assassinated mother, Rajiv Gandhi expressed hope that the situation would be discussed shortly by a Nonaligned Movement bureau in New York. He called for an easing of tensions and resumption of negotiations so that Central American countries could "decide their own future free from outside intervention and interference."

[Vice President R. Venkataraman announced selection of Indira Gandhi as this year's winner of the Jawaharlal Nehru Award for International Understanding, The Associated Press reported.]