Marrauding tribesmen killed at least 250 persons in 15 villages in the far northeastern Indian state of Assam in what is growing into the bloodiest election in India's history, according to reports reaching here today.

Four columns of Indian Army troops moved into the Nowang district of central Assam, where the fighting occurred Friday night and Saturday, and troops were put on the alert throughout the state as voters prepared to go to the polls today.

The vote represents the final round of balloting in a state assembly election to end emergency federal rule imposed last March in the wake of unrest then.

The troops supplemented the approximately 50,000 security forces of the Central Reserve Police who were airlifted to Assam two weeks ago to control unrest.

Officials in Gauhati, the state capital, confirmed the death toll in the series of raids of tribesmen armed with bows and arrows and machetes, but provided few details because of the remoteness of the affected area. Roads and bridges to many of the villages have been cut by the tribal raiders.

Foreign journalists have been prohibited from traveling to Assam, but Indian reporters there said that many of the victims were women and children and that many of the bodies had been mutilated.

Reports from Assam said that paddy fields between Nellie, on the national highway, and the raided villages were strewn with hundreds of wounded villagers.

The victims have been indigenous Assamese suspected of sympathizing with a frequently violent three-year-long drive to expel nearly one million illegal aliens who over the years have immigrated to Assam from neighboring Bangladesh, but the clashes in Assam also have taken on a communal character with Moslems and Hindus pitted against one another.

The cumulative death toll in Assam during the 18 days of violence has not been ascertained because many raided villages have not been visited by Indian officials. But the total death toll is likely to be more than 600, according to reports from Gauhati.

Indian reports at Nellie said they counted more than 250 in four of the 15 raided villages that stretch north from Nellie.

The militant Assamese movement has been battling to protect the indigenous Assamese population's ethnic and cultural identity by striking from the election rolls nearly 4 million immigrants--mostly Moslem and Bengali-speaking refugees from Bangladesh--and expelling nearly one million of them to surrounding Indian states.

Prime Minister Indira Gandhi had come under increasing pressure to cancel or postpone the election, but she insisted that she was bound by India's constitution to end the "president's rule" that has governed Assam for nearly a year and establish an elected state government by next month.

The prime minister rejected appeals that she seek a consitutional ammendment extending federal rule in Assam, saying that the measure could not pass in India's parliament, even though the two major opposition party leaders indicated they would support it.

For Gandhi, the election became a referendum on her own political prestige and a test of her authority, and a last-minute cancellation could have compounded her mounting political problems in the wake of recent election losses in state elections in southern India.

She has now come under increasing fire by opposition leaders of Parliament, who have said that the elections in Assam and the subsequent tragedies there were the result of political expediency