The Reagan administration's decision to rearm Pakistan has torn the fragile fabric of an emerging detente between India and Pakistan, already weakened by widely differing views on the Soviet invasion of Afghanistan, knowledgeable Western and Asian diplomats here indicate.

While the nine-year-old process of "normalization" on the Indian subcontinent may not yet be tattered beyond repair, diplomats here report that it has suffered a severe setback as New Delhi apparently feels its predominant position in the region is threatened by the closer security ties between the United States and Pakistan.

While India has not replied to Pakistan's offer last week to begin talks on a nonaggression pact, Indians unofficially dismissed it as a propaganda ploy to win U.S. congressional support for the arms purchases rather than a serious bid to improve relations with India.

Prime Minister Indira Gandhi has told Parliament that India had to be careful of an earlier Pakistani offer of troop reductions lest it get "caught in any trap."

Even Pakistan's prompt and successful action in ending the highjacking of an Indian airlines plane to Lahore does not appear to have improved relations between the two governments, even though Indian newspaper editorials generally applauded Pakistan.

The distrust has gone so far that one of the most experienced Western diplomats in this Indian capital has predicted a "slightly better than 50-50 chance" of a war between India and Pakistan within two years. He told American reporters that he would not have considered such a prediction two years ago, when the process of normalization was in full flower.

"If a war starts," the diplomat said, "the impetus will come from India rather than Pakistan. Indira Gandhi just cannot tolerate a resurgent Pakistan which will constitute somewhat of a threat to India."

While the veteran diplomat cited Washington's decision to rearm Pakistan as increasing the risk of an Indian-Pakistani war, he said "the major responsibility" lies with the Soviet Union, which "triggered the U.S. reaction" by its December 1979 invasion of Afghanistan.

It is unclear here to what extent the regional considerations weighed on the Reagan administration's security planners when they decided to seek closer Pakistani ties, including arms sales.

From interviews over the past six months throughout South Asia and the Middle East with American diplomats concerned directly with formulating South Asia-Persian Gulf policies, it appears that their major preoccupation was the forging of what Secretary of State Alexander M. Haig Jr. has called a "strategic consensus," anchored at one end by Pakistan and stretching through the oil-rich gulf to Israel, Egypt and Turkey.

India, with its close arms-supply relations with the Soviet Union and its tolerant attitude toward Moscow's invasion of Afghanistan, clearly had no place in that policy. New Delhi is viewed by the Reagan administration as "reflexively pro-Soviet," said a senior American official.

The Indian government took a far less strident view of the danger to the subcontinent from Soviet troops at the Khyber Pass than did the Pakistanis. The then U.S. president, Jimmy Carter, pledged American support without being asked and the martial-law government of President Mohammed Zia ul-Haq began soliciting Islamic and nonaligned nations for a condemnation of the Soviet action and economic and military support for Pakistan, which suddenly had become a front-line state against Soviet expansion.

New Delhi, on the other hand, abstained twice in U.N. votes that condemmed the Soviet invasion and argued for quiet diplomacy without harsh attacks on Moscow.

In many ways, the Gandhi government over the 21 months since the Soviet invasion of Afghanistan appears to find a Pakistan rearmed by the United States a greater threat than Soviet troops at the Khyber Pass, less than a two-hour flight from here.

One veteran Western diplomat cited what he called Gandhi's "deep-seated suspicion against the United States" as a reason for this view.

"She really believes that the United States would like to see India less powerful, which is not true," he continued.

It is difficult for outsiders to understand why a country as militarily strong as India -- with the fourth-largest military force in the world -- should be frightened of smaller, weaker Pakistan.

The London-based International Institute for Strategic Studies, for instance, said last month that India's recent multibillion-dollar arms purchases preserve its military superiority over Pakistan, whose equipment is considered to be outdated despite purchases of U.S. weapons.

However, the relations between the two nations during the 34 years of their existence have been marked by a mutual mistrust that often defies logical analysis.

Gandhi told an Australian newspaper that the arms sale was "the first step" of the Reagan administration toward securing military bases in that country. Pakistan's martial-law government strongly denied offering any bases to the United States.

Denial or not, the perception here is that the United States has gained a foothold on the subcontinent that threatens India.

The Western diplomat with experience in the region over two decades said India particularly objects to the reentry of the United States in subcontinental affairs. It pulled away from the region after the Nixon administration's famous 1971 "tilt toward Pakistan" failed to prevent the Indian-supported splitting of East Pakistan from the rest of that country to form Bangladesh.

That dismemberment of Pakistan into two separate states effectively insured Indian hegemony over the region.

The Western diplomat here argued that Indian hegemony promotes a stable South Asia and because of this country's size, military strength and economic vitality "is just as natural" as American hegemony in North America.

Nevertheless, Pakistani diplomats say India now feels frustrated because its dominance of the region is threatened by their country's success -- thanks to the Soviet invasion of Afghanistan and two good harvests -- in climbing out of political isolation and economic stagnation.

Besides building a new security relation with the United States and getting debts to other Western nations rescheduled, Pakistan has emerged as a power in the Islamic world and a counter to India in the nonaligned movement.

India appears bent on thwarting that. It has mounted a diplomatic campaign to keep Pakistan from being invited to rejoin the Commonwealth at its meeting this week in Australia. Pakistan quit in 1972 when Bangladesh joined.

Pakistan has been lobbying with Australia and Britain for an invitation to rejoin, but Gandhi has made it clear in interviews over recent weeks that she sees no reason to ask the Zia government back.

According to sources here, the government has gone to African and other Asian Commonwealth members to complain that Pakistan repeatedly uses international forums to raise bilateral issues -- particularly those with India.

There have even been hints in the press here that a concerted campaign to bring Pakistan back into the Commonwealth could lead to India's withdrawal.