Japan's prime minister used the 53rd anniversary of the bombing of Hiroshima Wednesday to urge India and Pakistan to stop their nuclear weapons testing programs.

"It is very regrettable that both India and Pakistan carried out nuclear tests in May," said Premier Keizo Obuchi, who took office last month and presided over the solemn annual ceremony marking the world's first atomic attack.Speaking to tens of thousands of people in Hiroshima - including the Indian and Pakistani ambassadors - Obuchi said the Japanese government strongly urged both countries to immediately stop nuclear testing, to unconditionally sign the comprehensive test ban treaty and to halt development of nuclear weapons and the missiles that carry them.

India and Pakistan have come under intense international criticism - and have received sanctions - for carrying out nuclear tests despite international efforts to curb nuclear proliferation.

Ceremonies marking the anniversary of the U.S. bombing of Hiroshima began with the offering of flowers at Peace Park, built at what was the epicenter of the blast, followed by a moment of silent prayer at 8:15 a.m. - the exact moment the bomb exploded above the city on Aug. 6, 1945.

As in most years, the moment of silence was broken only by the ringing of a Buddhist bell and the constant chirping of cicadas.

Many among the estimated crowd of 50,000 clasped their hands in prayer. Others held Buddhist prayer beads.

Hiroshima for the first time invited ambassadors to Japan from the five major nuclear powers - the United States, Britain, China, France and Russia - as well as those from India and Pakistan, to attend the ceremony.

But only India's Sidharth Singh and Pakistan's Touqir Hussain attended, said Mayumi Tochino of the Hiroshima city hall.

Addressing the crowd in Hiroshima, Mayor Takashi Hiraoka said the nuclear tests by those two countries raised tensions in South Asia and shook the foundation of the world's non-nuclear-proliferation movement.

"The people in Hiroshima feel a strong rage against the nuclear tests by the two nations, and worry about inducing a chain reaction of nuclear competition," Hiraoka said.

About 140,000 people were killed by Hiroshima's atomic bombing as well as the firestorm and radiation that ensued in the western city, which was a major military center during World War II.

Three days after the Hiroshima attack, the United States dropped a second atom bomb on Nagasaki, on Japan's southern main island of Kyushu, killing 70,000 people. Japan surrendered unconditionally on Aug. 15, 1945.

Hiroshima rebuilt after the war - except for the wreckage of one prominent building left on display to remind visitors of the massive destruction. Hiroshima, 425 miles southwest of Tokyo, is now a thriving commercial center of 1.1 million people.