Backed by an electoral mandate unprecedented in India's history, Prime Minister Rajiv Gandhi prepared today to constitute a government to his liking, saying his most immediate challenge will be to restore communal harmony and ensure equitable treatment of minorities.

Meeting reporters for the first time since his stunning landslide victory at the polls, the 40-year-old successor to the political dynasty started by his grandfather, Jawaharlal Nehru, and sustained for most of the past 18 years by his mother, Indira Gandhi, said his first task will be to strengthen the Indian union, strained by the assassination of his mother two months ago.

He attributed his overwhelming victory to a belief by more than 200 million Indian voters that the ruling Congress (I) Party alone is capable of keeping the country united and "because they want to look to the future." Following his pro forma reelection as leader of his party's parliamentary caucus and swearing in again as prime minister of the world's most populous democracy, Gandhi is expected to form a new government, possibly as early as Monday evening, Congress (I) sources said.

Yesterday Gandhi amassed an electoral victory of proportions never seen before in India, even during the heyday of his grandfather soon after independence or at the height of Indira Gandhi's popularity.

With 64 parliamentary districts still to be counted, the Congress (I) Party won 394 of 494 seats decided so far in the Lok Sabha, or governing house of Parliament -- slightly more than 80 percent of the seats.

In the process, he virtually wiped out the established national opposition parties, leaving only 10 seats to the Janata Party, which ousted his mother in 1977 following her Draconian "emergency" era and which governed India for 33 turbulent months. Even Janata Party President Chandra Shekhar was defeated. The party had held 21 seats in the Lok Sabha.

For the first time in Indian history, Congress (I) polled more than 50 percent of the total votes cast, far more than the 43 percent that Indira Gandhi won during her dramatic political comeback in the 1980 national election. The previous high was 49 percent, amassed by Nehru in 1957.

On the eve of its 100th anniversary next month, the party found iself more entrenched than ever in Indian political life as the result of the mandate given to the once retiring, self-effacing surviving son of Indira Gandhi. Rajiv Gandhi, a newcomer to politics, reluctantly gave up his job as an airline pilot three years ago to take up the mantle of his brother, Sanjay, after Sanjay's death in a stunt-plane crash.

With returns from remote election districts still coming in, Congress (I) was on the verge of winning nearly 400 seats in the eighth Lok Sabha, which convenes next month. Only 509 of the 542 elective seats in the house were contested in the three-stage election because civil unrest had forced the suspension of balloting in the states of Punjab and Assam. Two additional appointed seats are reserved for Anglo-Indians.

Going into the election, the Congress (I) Party, formed in 1979 when Indira Gandhi split the Congress Party after her defeat by the Janata Party, held 355 seats in the Lok Sabha, or People's House.

Rajiv Gandhi, in his first major electoral campaign, rolled up a 300,000-vote plurality in the Amethi district of the northern state of Uttar Pradesh, burying the feckless political career of his estranged sister-in-law, Menaka Gandhi. The 29-year-old widow of Sanjay formed an opposition party after being expelled from the family home by Indira Gandhi during a family feud.

Congress also virtually eliminated the splinter opposition Bharatiya Janata party, which had held 16 Lok Sabha seats, sending to defeat its president, former Janata government foreign minister Atal Bihari Vajpayee, in Madhya Pradesh's Gwalior district and leaving the party only two parliamentary seats.

The Communist Party of India (Marxist), which had led the opposition with 36 seats in Parliament, had accumulated only 14 seats by tonight, mostly in Communist dominated West Bengal, although its total was expected to rise somewhat by the time final results are tallied.

The regionally based Telegu Desam Party, founded by Andhra Pradesh Chief Minister N.T. Rama Rao, emerged from the election as the dominant national opposition party, winning 27 parliamentary seats from the south Indian state. Rama Rao is a former matinee idol whose popularly elected state government Indira Gandhi unsuccessfuly tried to topple last August.

The rout of the Congress (I) in Andhra Pradesh, which had held 37 of the state's 42 parliamentary seats, thrust Rama Rao into a national opposition leadership role, even though he did not even run for Parliament.

Another regionally based party, the Anna Dravida Munnetra Kazhagam party of the southern state of Tamil Nadu, captured 12 seats in Parliament.

Congress handed such a humiliating defeat to the Janata-led government of Ramakrishna Hegde in Karnataka that the state government resigned and announced that it would dissolve the state assembly and hold new elections. Gandhi's party also made impressive inroads in the Communist citadel of West Bengal, where it had only four seats in the last Lok Sabha session.

Congress also won all seven seats in the union territory of New Delhi, considered a bastion of the Bharatiya Janata Party.

Nationally, the Congress (I) Party virtually eliminated the Lok Dal (People's Party) of Charan Singh, who served briefly as prime minister following the collapse of the Janata coalition in 1979. Although Singh won reelection to Parliament, his party, renamed the Dalit Mazdoor Kisan Party (Revolutionary Farmers and Workers Party) won back only two of the Lok Dal's 25 parliamentary seats.

At his press conference at the prime minister's residence, where Indira Gandhi was slain two months ago, Rajiv Gandhi said that India's foreign policy would "very substantially be the same" as his mother's and that he would seek a dialogue with Pakistan, with which India has fought three wars during the past 37 years.

Gandhi said he would address the nation in the first week of January, after the official declaration of the election results.

Political analysts here said they expected the prime minister to form a new government quickly in order to avoid lobbying from aspirants to ministerial posts. The analysts also said that given the size of his electoral mandate, Gandhi also would be expected to drop some members of the Cabinet that he inherited from his mother.