A parliamentary ally of Prime Minister Rajiv Gandhi was assassinated here this morning, along with his wife and a party worker, after he had been warned by Gandhi's office that he was on a terrorist hit list because of his role in anti-Sikh riots last November.

Two gunmen fired at least 15 shots from a Sten gun and a revolver at the unguarded house in which Lalit Maken, 34, had been meeting constituents before going to Parliament. The gunmen and a lookout escaped on a stolen motor scooter.

Maken's brother-in-law, Deepak Rai, told reporters outside the hospital that the first-term member of Parliament had been warned by Gandhi's office two months ago that he was on a terrorist hit list. United News of India quoted intelligence sources here as saying that Maken had received a threatening letter a few days ago. Nonetheless, the news agency reported, a police guard had been withdrawn six days ago.

Delhi Police Commissioner Ved Marwah said Sikh terrorists were a strong possibility as the assassins. But he said police were also investigating the possibility that Maken was killed as a result of rivalries either within the ruling Congress (I) Party or in the trade union movement that was his power base.

Marwah announced tonight that police had picked up three young men for "sustained interrogation" in connection with the slaying.

Maken's assassination came just a week after Gandhi reached an agreement with the mainstream Sikh political party, the Akali Dal, to settle many Sikh demands for greater autonomy in the vital state of Punjab, India's granary, situated on the strategic western border with Pakistan.

Extremists seeking a separate Sikh nation in Punjab threatened Saturday to intensify their campaign of violence in an effort to scuttle the agreement, which they condemned as a sellout.

It appears, however, that the accord between Gandhi and Akali Dal leader Sant Harchant Singh Longowal has received widespread support from middle-of-the-road Sikhs here and in Punjab who are tired of more than three years of violence by the extremists. More than 2,000 persons have been killed in the Punjab violence.

In addition, Sikh extremists were accused by the FBI of mounting an assassination plot against Gandhi during the prime minister's American visit in June and they are suspected of having placed explosives in an Air-India jumbo jet that crashed off the coast of Ireland, killing all 329 persons on board.

Speaking to a stunned Parliament this morning, shortly after Maken's assassination was announced, Gandhi appealed for an end to "the cult of violence springing up all around us." He said there are "certain elements" who are trying to hold "the entire society ransom."

Maken was the first member of Parliament to be killed since Gandhi's mother, Indira Gandhi, then prime minister, was assassinated Oct. 31 in the garden of her official residence by two Sikh members of her security guard.

Indira Gandhi's assassination sparked a bloodbath against Sikhs in northern India that has been called the worst violence since that which accompanied this country's independence and partition into Hindu India and Moslem Pakistan nearly 38 years ago. More than 2,700 people were killed in the riots that followed Indira Gandhi's death.

A private study by the People's Union for Democratic Rights and the People's Union for Civil Liberties named Maken as one of the officials of the ruling Congress (I) Party who were identified by victims as having instigated the violence against Sikhs.

Maken, according to the allegations, "reportedly paid to mob RS 100 [$10] each plus a bottle of liquor," the report stated. In addition, it said, "instructions to mobs indulging in arson were given from inside" a car owned by Maken, who was not then a member of Parliament. That report has been widely circulated among Sikhs, who blame Congress (I) leaders for organizing the violence.

Maken was a powerful labor leader here, head of the Delhi Transport Corp. workers union with a large following among bus drivers and conductors, who called a short strike this afternoon when they learned of their leader's death.

The killers were said to be in their twenties, and first reports said they were clean shaven. Most Sikhs wear beards as part of their religion.