As 100,000 mourners attended the cremation of assassinated Sikh political leader Harchand Singh Longowal in a Punjab village 130 miles northwest of this capital, Indian opposition leaders issued a joint call for postponement of the Punjab State elections scheduled for Sept. 22.

Longowal was shot fatally yesterday as he addressed supporters after launching the election campaign of his mainstream Sikh party, the Akali Dal, at a rally in the Punjab village of Sherpur. The assassins appeared to be Sikh extremists opposed to the accord he signed with Prime Minister Rajiv Gandhi last month, paving the way for the state's first elections in two years.

Curfews were imposed in parts of Punjab and the Indian Army was put on alert to prevent fresh violence in the Sikh-majority state, where more than 2,000 persons have been killed in separatist fighting during the past three years. There were reports of scattered violence there, including the shooting death of a village official by two motorcycle-riding Sikhs as he distributed relief aid to monsoon flood victims.

Across the state, businesses were closed by a one-day strike to protest Longowal's assassination.

Gandhi called an emergency meeting of his Cabinet. "Longowal was a man of faith, sincerity and dedicated to national unity and the development of Punjab," Gandhi told Parliament, which adjourned in tribute for the day after hearing other eulogies. "We will fight terrorism with all our might," the prime minister vowed.

Sikh political leaders joined the opposition in calling for postponement of the state elections. The opposition leaders' statement called Longowal's death "a severe blow to the atmosphere of peace and harmony that was generated by the Punjab accord" he had signed with Gandhi on July 24.

"We are in a state of mourning," said one Akali Dal leader, Gurcharan Singh Tohra. "We can't even think of elections in the state."

The federal Election Commission called the state's senior officials to New Delhi to reassess whether the elections should be held as scheduled. The chief election commissioner, R.K. Trivedi, said that "postponement will be no problem."

But Satindra Singh, editor of Punjab's main English-language daily, the Tribune, and a friend and adviser of Longowal's, warned that "if he [Gandhi] postpones elections now he will be accused of submitting to extremists. If not," Singh continued, "there will be another Assam-in-1983-like situation in Punjab."

In 1983, more than 3,000 persons were killed in violence in northeastern Assam State after Gandhi's mother, Indira Gandhi, insisted on holding elections that local militants tried to block.

Defense Minister P.V. Narasimha Rao and Punjab Gov. Arjung Singh were among the mourners at Longowal's funeral pyre, where more than 500 soldiers held back a crowd of Sikhs and Hindus that police estimated at 100,000. The pyre was lit by Jathedar Kirpal Singh, head priest of the Sikhs' holy Golden Temple in Amritsar, in another sign of Longowal's standing in the Sikh community.

The 53-year-old Longowal's body, draped in white and saffron cloth and carried on a vehicle wreathed with garlands, was brought to Longowal, the Punjab village whose name he took, from the city of Sangrur, where he died in a hospital after the shooting. About 20,000 mourners marched for five hours in the funeral procession as troops lined the 10-mile route and bystanders, estimated by police as another 100,000, threw marigolds and pieces of red and gold cloth, symbolizing martyrdom, at the slain leader's cortege.

In the capital, hundreds of Sikhs and some Hindus visited Sikh shrines, offering prayers for Longowal and for peace in Punjab. "It is a time of sorrow for us," said a Sikh businessman, Ajaib Singh, as he closed his shop here in mourning. "We will wait and watch."