With more than half the votes counted today, a recently formed party of native Assamese scored impressive election gains over Indian Prime Minister Rajiv Gandhi's ruling party in Monday's statewide elections in the northeastern state of Assam.

With declared results in for 78 of the 125 contested seats in the state assembly, the Assam People's Council won 42 seats, Gandhi's Congress (I) Party won 17, and a minority group, the United Minorities Front, won six seats. The rest of the declared seats went to independent candidates and minor parties.

The results so far leave Gandhi's supporters, who have controlled the ethnically divided state of Assam for all but two years since India gained independence in 1947, with only a slim chance of retaining power through a coalition.

The Assam People's Council, formed in October, is dominated by ethnic Assamese Hindus who want a crackdown on illegal immigration by mostly Moslem immigrants from neighboring Bangladesh. The racial and ethnic tension between the two groups has resulted in six years of bloody turmoil and prompted Gandhi to call midterm state elections here as part of a settlement to the turmoil.

The party needs a simple majority of 64 seats in the 126-seat state assembly to form a government in this oil-rich, tea-growing state. (One race was postponed because of the death of a candidate.) As results were coming in late today, the Assamese party's general secretary, Bhrigu Phukan, predicted "a sweeping victory" and "80 seats for sure."

Also at stake were 14 seats in India's Parliament. The Assam People's Council was reported leading in six and Congress (I) in three.

Assam Chief Minister Hiteswar Saikia, head of the incumbent Congress Party state government, appeared to concede defeat: "We accept the people's verdict," The Associated Press reported.

Coming so soon after the defeat of Gandhi's Congress Party in the September election in the northern state of Punjab and in other state elections on Monday, the results so far from the Assam election indicate that the ruling party is losing some of its political control.

At the same time, however, it may be a price Gandhi is willing to pay; local observers say the election of a locally popular government may bring an end to the violence that has claimed more than 3,600 lives in communal violence.

When N.T. Rama Rao, a former movie star and the chief minister of the southern Indian state of Andhra Pradesh came to Assam to campaign for the Assam People's Council, many political analysts saw the move as a convergence of opposition parties against the ruling Congress Party.

The Congress Party also suffered setbacks in nine other states in elections to fill state assembly and national Parliament vacancies. Congress Party candidates were defeated in West Bengal, Bihar, and Orissa states but were victors in Uttar Pradesh, Rajasthan and New Delhi.

The manual vote-counting in Assam has taken an unusually long time because of the record turnout of more than 70 percent of the 9.9. million eligible voters in this state. By comparison, only 17.4 percent of voters took part in Assam's last general state elections in 1983.

As part of the settlement in Assam, Gandhi also agreed to expel all illegal immigrants who entered Assam from Bangladesh after 1971, and to disenfranchise for 10 years those settlers who came between 1966 and 1971, after which they are to become full Indian nationals.

The mainly Hindu natives of Assam complain that millions of Moslem immigrants from Bangladesh threaten to make them a minority in their own homeland. Bengali immigrants make up about 40 percent of the state's 22 million inhabitants.