India was harshly condemned around the globe yesterday for conducting nuclear tests on Monday. Arch-rival Pakistan debated whether to respond with a nuclear test of its own, and Japan, India's biggest aid donor, said it may withhold assistance worth as much as a billion dollars.

In Washington, President Clinton said he was disturbed by an act that threatens the stability of South Asia and "directly challenges the firm international consensus to stop nuclear proliferation." He vowed "to implement . . . fully" U.S. sanctions laws intended to deter the spread of nuclear arms and urged India to promise it would carry out no further tests.

In addition, the United States recalled its ambassador to New Delhi, while the White House said that Clinton's planned trip to India later this year is under review.

Indian Prime Minister Atal Bihari Vajpayee announced Monday that his country -- the world's second-most-populous -- had staged an underground test of three nuclear devices, including a thermonuclear one. It was the second nuclear test in India's history; after the first, in 1974, India had pointedly described its nuclear effort as peaceful, but an aide to Vajpayee said after Monday's exercise that India "has a proven capability for a weaponized nuclear program."

The tests prompted an immediate outcry from neighboring Pakistan, which is widely assumed to be capable of producing nuclear weapons, as well as India. Pakistan, which has fought three wars with India in the past 50 years, has never tested a nuclear device. But Pakistani Foreign Minister Gohar Ayub Khan declared today that the "nuclear race started by India will be reciprocated," while the nation's political and military leadership debated whether a more measured response should be undertaken.

Pakistani Prime Minister Nawaz Sharif, returning from a trip to Central Asia, met for several hours with senior military officials and senior members of his government to discuss India's action, which appeared to have taken Pakistan's security establishment by surprise. "We didn't have any advance information on these explosions," said a member of Sharif's cabinet.

Another cabinet member said, "Not surprisingly, many ministers thought it was the ideal moment for Pakistan to test its nuclear device," and Pakistan's army informed Sharif that it will be ready "within a week" to conduct an underground nuclear test on 24 hours' notice. But officials familiar with today's deliberations spoke of a division within the cabinet over an appropriate Pakistani response.

According to an aide, Sharif appeared to favor "a balanced and moderate response" and ordered a report on the cost the country would have to bear if a Pakistani nuclear test brought international sanctions.

The influential army, which ruled Pakistan from 1977 to 1988 and remains politically powerful, has not openly backed the demand for nuclear testing. "The army has left it up to the prime minister to make a decision that suits the national interest. The army will stand by his decision," a senior army official said.

The United States, which said Monday that it was "deeply disappointed" by India's action, yesterday ordered Ambassador Richard Celeste, who was en route to India from Hawaii, to return to Washington for urgent consultations.

Clinton made a direct appeal to Pakistan, China and other nations in the region "not to follow suit -- not to follow down the path of a dangerous arms race."

"I want to make it very, very clear that I am deeply disturbed by the nuclear tests which India has conducted, and I do not believe it contributes to building a safer 21st century," Clinton said at the White House.

White House Press Secretary Michael McCurry suggested that India's actions had thrown into question Clinton's plans to travel there. "We certainly are going to have to assess that development as we consider our itinerary," McCurry said.

Secretary of State Madeleine K. Albright recommended that Clinton implement sanctions against India immediately, spokesman James Rubin told reporters. "Secretary Albright believes it was appalling that Indian diplomats left the administration with anything but the impression that there would be nuclear tests this week," Rubin said.

Clinton discussed the Indian tests by telephone with Russian President Boris Yeltsin, who said "India has let us down with its explosions." Russian Foreign Minister Yevgeny Primakov said, however, that Moscow was unlikely to impose sanctions, news services reported.

Japan, the only country ever to suffer nuclear bombings, protested India's nuclear tests by threatening to withhold grants, low-interest loans and other assistance. Officials said they were still deciding how extensively they would cut the billion dollars in aid Japan supplies to India. Government officials said they were reviewing their entire diplomatic stance toward India.

Japan took similar action against China when it tested a nuclear weapon in 1995, withholding more than $50 million in grants for almost two years. Officials said that the amount withheld was relatively small in China's case because officials feared reversing China's trend toward more engagement with the rest of the world.

Prime Minister Ryutaro Hashimoto condemned the Indian tests as "extremely regrettable." Hashimoto said he had sent a letter in March to Vajpayee in which he said, "I am carefully watching the trend of the nuclear policy of India, which has a great influence to the peace and stability of the world." Hashimoto said he was still waiting for a response, and "then they just did it."

The mayors of Hiroshima and Nagasaki, the two cities flattened by U.S. nuclear blasts at the end of World War II, sent letters of condemnation to India, and demonstrators in those cities and in Tokyo took to the streets in protests that were both angry and mournful.

"I can't express in words what a shock it is," said Nagasaki Mayor Itcho Ito, who called the three underground tests "an outrage that ravages the wishes of the victims and citizens" of his city. In Hiroshima, where demonstrators staged a sit-in protest at the atomic bomb memorial, survivor Akito Suemune, 71, called India's action "strictly unforgivable."

After a 24-hour silence, China today joined the worldwide condemnation of its Asian neighbor. "The Chinese government expresses grave concern about India conducting nuclear tests," state radio quoted Foreign Ministry spokesman Zhu Bangzao as saying. "India's conducting of nuclear tests runs against international trends and is detrimental to the peace and stability in South Asia." Khan reported from Karachi, Pakistan, and Sullivan from Tokyo. CAPTION: Pakistani leader Sharif is said to favor "moderate response" to tests.