A three-year, occasionally violent ethnic movement in the far northeastern state of Assam to expel nearly a million illegal immigrants from neighboring Bangladesh is headed for a showdown next month with the government of Prime Minister Indira Gandhi.

The student-led movement has vowed to block an election called to replace extraordinary central government rule with a new legislature and to disrupt life in the remote but economically important Indian state in an effort to protect the indigenous Assamese population's ethnic and cultural identity from the immigrant influx.

The new campaign could cause national economic disruption, and it threatens to add more turmoil to India's troubled northeast as the ethnic protectionists try to regenerate the massive popular support that their protest commanded at its apex in 1980.

However, Gandhi's government, bound by the Indian Constitution to restore legislative rule that was dissolved in March and replaced by an emergency "president's rule," says that it will go ahead with state assembly elections anyway and provide as much security as needed.

Already, scores of militant students have been arrested under national security regulations, and the balloting has been scheduled to be spread over three days beginning Feb. 14 to allow for the deployment of security forces.

State administrators appointed by New Delhi have placed police forces on alert and have ordered preventive arrests to maintain order.

Three political parties in opposition to Gandhi's ruling Congress-I Party have said they will boycott the vote, and the leader of the student protest predicted that only 5 percent of Assam's voters will turn out.

While the genesis of the conflict can be traced back to a flow of immigrants to Assam from the eastern part of then undivided Bengal that began at the turn of the century, the bulk of what the Assamese call "foreigners" arrived in three waves, beginning in 1947 when the Subcontinent was partitioned into India and Pakistan; in 1965 during the Indo-Pakistani war over Kashmir; and since 1971, when the new nation of Bangladesh was created out of war-torn East Pakistan.

The indigenous Assamese--a distinct ethnic and linguistic group that traces its origins to the ahom nomads from Thailand in the 12th century, but who are primarily Hindu--say they are in danger of being culturally and economically overrun by the influx of mostly Moslem immigrants across the porous border from what is now Bangladesh.

Precise figures on the number of illegal aliens in Assam are hard to come by, and depend upon what definition is used to describe indigenous Assamese. Western journalists are prohibited from entering the irregularly shaped state that is connected to the rest of India by a narrow neck of land at the northeast corner of Bangladesh. Nonetheless, estimates range from between 800,000 and 967,000 Moslems and Hindus who settled illegally between 1961 and 1971, of whom 215,000 have refugee status.

Mindful that Bangladesh has refused to accept the immigrants, the student agitators have demanded that all "foreigners" who entered after 1961 be relocated in other parts of India, while the government has countered by saying that it would expel only those who settled illegally after 1971.

At its peak in 1980, the Assam unrest had turned into one of the largest mass demonstrations since the "Quit India" movement that ended with independence from Great Britain in 1947. The flow of oil from Assam to the rest of India was interrupted, and occasional bomb blasts rocked the state, including one in the capital of Gauhati that killed 20 persons. By official estimates, the three-year protest has cost nearly $1 billion in lost oil and industrial production. Assam provides about 30 percent of India's domestic oil production.

Since 1980, the intensity of the protest has waxed and waned, but popular support for the movement was sufficient to force Gandhi's government into protracted and ultimately unsuccessful negotiations after an attempt to crush the student-led agitation resulted in massive defiance of a ban on public meetings.

Protest leaders charged that local Congress-I (for Indira) Party officials tacitly encouraged immigration from economically blighted Bangladesh, assisted the aliens' settlement and then put their names on the electoral rolls to gain political support. They claim that 2.3 million illegal immigrants entered Assam between 1971 and 1979 to escape hunger and political instability and that the influx is continuing.

P. P. Nayar, Indian Home Affairs Ministry special secretary for Assam, ridiculed the students' claims, saying in an interview that there was no historical precedent in the world for such a mass expulsion.

"Can you imagine expelling all the Mexican workers from the southwestern United States?" he asked.

Humanitarian and international restraints aside, Nayar also cited Indian legislation governing citizenship for persons of Indian origin and emigrants fleeing East Pakistan.

Nayar said there is no way of knowing how many immigrants are in Assam, but he estimated that 60 percent of the current population is Assamese-speaking. Other estimates by independent demographers put the number of ethnic Assamese at between 9 million and 12 million.

More effort is needed, he said, to assimilate the Bengalis in Assam and at the same time overcome fears by the ethnic Assamese that they are economically threatened.