A Delhi judge convicted three Sikh men of murder and conspiracy today and sentenced them to death for the October 1984 assassination of prime minister Indira Gandhi.

In a heavily guarded courtroom inside Delhi's main jail, Judge Mahesh Chandra ended the eight-month-long trial by delivering his verdict from behind a bulletproof glass shield. Each of the defendants -- two former policemen assigned to Gandhi's security detail and a former government clerk -- maintained his innocence, and defense lawyers immediately promised an appeal.

With much of northwestern India under tightened security this week because of tensions surrounding the Sikh community, there were immediate fears that the judgment would further inflame Sikh extremists who have mounted a campaign of terrorist attacks in the Sikhs' home state, Punjab.

Punjab and neighboring Haryana State remained tense as a central government commission prepared to rule on a territorial dispute between them arising from last July's compromise settlement of Sikh political and economic demands. Police in the two states have made more than 700 "preventive arrests" in the past two days, while paramilitary forces were deployed in the region and airport security tightened throughout India.

Sikh extremists, who oppose the compromise between moderate Sikhs and Prime Minister Rajiv Gandhi, openly describe Indira Gandhi's assassination as a heroic act of revenge after she ordered the Indian Army to storm the Sikhs' most sacred shrine in June 1984 to flush out armed extremists.

Chandra, a veteran judge who has tried more than 100 murder cases but had never before levied the death penalty, emphasized that two of the defendants, 22-year-old Satwant Singh and 43-year-old Balbir Singh, had been assigned to protect Gandhi's life.

"I am of the opinion that this offense is of the rarest of rare nature in which extreme penalty of death is called for," Chandra said. There are no jury trials in India, but imposition of the death penalty must be confirmed by a higher court.

Chandra convicted Satwant Singh of shooting Gandhi as she walked from her home to her office to tape a television interview. Another Sikh bodyguard, Beant Singh, also shot Gandhi but was killed on the spot by other security agents.

Chandra found Balbir Singh and 50-year-old Kehar Singh guilty of conspiracy in the killing. The three defendants are unrelated, but -- as is customary among Sikhs -- use the name "Singh," which means "lion."

The chief lawyer for Satwant Singh, Pran Nath Lekhi, said the defense will appeal the verdict to the Delhi High Court within a week. "If any judgment is worth being struck down, this particular one is," Lekhi said. "We are confident it will not stand."

The guilty verdict had been widely expected; even some of the defendants' relatives had appeared pessimistic before Chandra announced his decision. The death penalties caused surprise, and raised concerns about violent reaction from Sikh extremists.

Many in the Sikh community have criticized the central government for failing to prosecute Hindus who were alleged to have incited anti-Sikh riots following Gandhi's assassination, and it was feared that today's death sentences would heighten a feeling among Sikhs of unequal justice for Sikhs and Hindus.

Such Sikh anger might combine with the current tensions in Punjab State to endanger the progress toward resolving India's 3 1/2-year-old Sikh crisis, according to retired Gen. Jagjit Singh Aurora, a leading moderate Sikh spokesman.

Aurora agreed with a widespread view that the situation in Punjab is at its most sensitive since last fall's state elections, which installed the main Sikh party, the Akali Dal, in power. The moderate Akali government has been pressured this month by protests and terrorist attacks of extremists calling for the release of detained Sikh militants.

Many observers fear that either Punjab's Akalis or the Congress (I) party government in neighboring Haryana will be undercut politically, whichever loses the current boundary dispute over 54 villages in southwestern Punjab. The central government commission preparing a recommendation on the issue asked today for two more days to finish its report.

Although Gandhi's assassination was an intensely emotional event for India, the trial moved out of the headlines soon after it started last May. Several early conspiracy theories, which had suggested foreign involvement in the killing, died for lack of evidence, and the trial's most spectacular moments were provided by the tactics of defense lawyer Lekhi, a leading member of the opposition Bharatiya Janata Party.

At several points in the trial, Lekhi argued that members of Gandhi's family -- including her son and successor, Rajiv Gandhi -- might have conspired to kill her, suggestions that Judge Chandra ruled "irrelevant."