Mr. Mishra said the tests would help scientists design ''nuclear weapons of different yields for different applications and for different delivery systems'' -- meaning, Indian experts said, that the explosions were meant to test different types of nuclear warheads for India's fast-developing missile program, which has a mix of delivery vehicles to reach targets as close as Pakistan and as distant as China.

The tests were widely welcomed in India, with hardly any immediate dissent from opposition political parties and little sign of the Gandhian pacifism that was a strong element in Indian policy in the early years after independence in 1947.

Even Mr. Vajpayee's predecessor as Prime Minister, I. K. Gujral, a moderate who blocked the tests during his year in office, said: ''It was always known that India had the capability to do this. The tests only confirm what was always known.''

But the outcry from outside India was almost universal, with dozens of governments expressing anger that India had broken an informal moratorium on nuclear testing that went into effect in 1996, when India and Pakistan stood aside as scores of other nations met at the United Nations to endorse the Comprehensive Test Ban Treaty, which prohibits all nuclear tests. The treaty is widely regarded as a key step toward halting the spread of nuclear weapons.

The Indian tests drew immediate condemnation from the Clinton Administration, which said the United States was ''deeply disappointed'' and was reviewing trade and financial sanctions against India under American nonproliferation laws; from other Western nations, including Britain, which voiced its ''dismay'' and Germany, which called the tests ''a slap in the face'' for 149 countries that have signed the treaty, and from Kofi Annan, the United Nations Secretary General, who issued a statement expressing his ''deep regret.''

But perhaps the most significant reaction came from Pakistan, which raised fears that years of effort by the United States to prevent an unrestrained nuclear arms race on the subcontinent were on the verge of collapse. In the absence of Prime Minister Nawaz Sharif, who was visiting Central Asia, Foreign Minister Gohar Ayub Khan hinted that Pakistan, which has had a covert nuclear weapons program since the early 1970's, would consider conducting a nuclear test of its own, its first.

''Pakistan reserves the right to take all appropriate measures for its security,'' Mr. Ayub Khan said in a statement to the Senate in Islamabad, the capital, that came amid demands from right-wing politicians and hard-line Islamic groups for an immediate nuclear test.