India's fragmented opposition parties have joined in their first coordinated challenge to Prime Minister Indira Gandhi's government.

Observers give the opposition no chance of unseating the nearly two-year-old Gandhi government, which holds a two-thirds majority in Parliament, but they say the new unity of the more than dozen opposition perties is making the government pay at least lip service to their views.

Last week, they forced the legislature into extended debates on foreign and economic policies and on charges of massive administration corruption.

"The opposition can no longer be bypassed," said Atal Bihari Vajpayee, leader of the Bharatiya Janata Party and an opposition spokesman.

"We have succeeded in keeping the government on its toes," said Vajpayee, foreign minister in the previous, Janata coalition government. "Its failures have been highlighted. Its officials have been on the defensive on almost all issues."

Vajpayee said opposition statements had forced the Gandhi administration to take a more responsive tone toward Pakistan's offer of a nowar pact.

Furthermore, the opposition appeared united in criticizing the Gandhi government for obtaining a $6 billion loan from the International Monetary Fund. One opposition figure, Lok Dal party secretary Satpal Malik, called the loan "the biggest blot on the Indian economy."

Gandhi appears to have recognized the new power of a united opposition.

She has called opposition leaders to meet with her on two of India's more divisive issues. The first is the highly publicized although apparently tiny movement for a separate Sikh nation. The movement has disrupted politics in India's breadbasket state of the Punjab.

The second is the longstanding agitation in the oil-rich northeast state of Assam to protect the cultural identity of natives against an influx of Bengali "foreigners" from Bangladesh and from overcrowded nearby Indian states.

Gandhi's son, Rajiv, has decried the unity movement as an attempt to set up a "motley opposition" based on the coalition that overthrew the Gandhi government in 1977 elections. This coalition, however, fell apart two years later and allowed Gandhi's return to power in the elections of January 1980.

There is little chance that the noncommunist opposition parties will merge, although they have agreed to unite on specific issues.

Among these are stands against what some perceive as the government's pro-Soviet tilt; what is viewed as growing corruption in the country exemplified by charges that Gandhi's chief minister in Maharashtra State, A.R. Antulay, misused public funds; and government inability to stem growing violence between high-caste Hindus and untouchables.

Underlying all these issues is the view that Gandhi's one-person rule without any counterweight from an organized opposition threatens India's fragile democracy.

Observers say the event that precipitated the coalescing of opposition parties was the postponement in November of a district election in Uttar Pradesh, where voting irregularities blamed on proGandhi forces invalidated a June 15 poll. One candidate in that election was H.N. Bahuguna, a former Gandhi ally who has become a particular nemesis of hers.

"This, more than anything else, has spurred the opposition parties, right and left, to think seriously about joint action," said Kuldip Nayar, a political commentator. Still, he said, the diverse opposition lacks a univerally respected leader.

Squabbling goes on. Within the Janata Party, for instance, Chandra Shekhar, the party president, wants opposition unity while former prime minister Morarji Desai, who has assumed the role of opposition elder statesman, is against it.

Former prime minister Charan Singh, the Lok Dal president who brought down Desai's government, favors merger; Vajpayee opposes it in favor of "functional unity."

Vajpayee told reporters yesterday that "past experience, present distrust and future forebodings" make it unrealistic to talk of merger.

Furthermore, personal ambition still motivates opposition leaders.

Two of the brightest, for instance, Vajpayee and Subramaniam Swamy, were egged on by Congress-I (for Indira) members to carry on a feud in Parliament over whether Vajpayee had called Swamy a CIA agent.