Prime Minister Olof Palme, who became internationally prominent for championing Third World causes and opposing nuclear weapons, was assassinated by two gunmen in central Stockholm last night.

Palme, 59, was walking home from a movie with his wife, Lisbeth, when he was shot twice in the stomach at close range in front of several horrified witnesses.

As the prime minister lay in a pool of blood on the snowy sidewalk, two teen-agers ran to his aid.

A taxi driver alerted police and an ambulance via his radio as the two assailants escaped in a Volkswagen car heading north out of Stockholm at high speed.

Palme was rushed to the hospital, where he died 36 minutes later.

Lisbeth Palme also was hit by gunfire but was not seriously hurt. She left the hospital after treatment.

The Swedish government met in an emergency session under the leadership of Deputy Prime Minister Ingvar Carlsson.

"It is terrible," said Carlsson.

Ulf Adelsohn, leader of the opposition Moderate Coalition Party, said: "It is an almost unbelievable shock. It is a thing one cannot believe could happen in Sweden. It is a tragedy not only for Olof Palme's family but for the democratic ideal all Swedes support.

"Sweden will never be the same," he added. "The meaningless violence will always throw a shadow over our political life."

Inspector Sune Sandstrom of Stockholm police said several witnesses were being questioned.

"We have no clues as to who the attackers are or what their motive might have been," he said. "All available resources are being drafted."

Police said the two killers appeared to be in their forties. One was said to have dark hair and be wearing a dark coat.

Swedish police issued a nationwide alert that mentioned members of the Ustashi, a Croatian nationalist separatist group that committed numerous terrorist acts in the 1970s, including the assassination of the Yugoslav ambassador in Stockholm, United Press International reported. However, a police spokesman said later, "We have no indications that any political organization carried out the killing."

One of the Yugoslav ambassador's killers, Miro Baresic, was caught and sentenced to life imprisonment in Sweden. Baresic petitioned for his release last year but was turned down by the Swedish government after representations from Yugoslavia.

There was no bodyguard at the time of the killing, and calls are already being made here for increased security for public figures.

A shocked crowd gathered at the scene of the assassination, with many people weeping openly.

Swedish Radio broke off its usual all-night popular music program to announce the news of the assassination.

Classical music was played and regular news bulletins throughout the night kept a stunned nation informed.

Palme was serving his second term as prime minister. His Social Democratic Labor Party won overwhelmingly in an election on Sept. 19, 1982, ending six years of nonsocialist rule, and he formed a government Oct. 7, replacing Thorbjorn Falldin as prime minister.

Palme entered parliament in 1956 and served in a variety of Cabinet posts. He was prime minister the first time from 1969 to 1976, when his defeat ended 44 years of socialist rule in Sweden.

Palme had many political enemies in Sweden. He frequently was accused of breaking with the nation's "middle way" of consensus politics after coming to power in 1969.

He introduced a new, contentious style to Swedish political life, frequently launching into bitter invective against his opponents in the parliament, or Riksdag.

He was an outspoken critic of U.S. involvement in the Vietnam War and in 1968 took part in an antiwar demonstration in Stockholm with the North Vietnamese ambassador to Sweden.

He once likened former president Richard Nixon's 1972 order for the bombing of Hanoi to acts committed by Adolf Hitler. Palme was Sweden's only international politician and was particularly active in the peace and disarmament movement. He was a U.N. mediator in the Persian Gulf war between Iran and Iraq.

He had been criticized by the Reagan administration for his statements condemning U.S. involvement in Central America.

But he also frequently had attacked the Soviet invasion of Afghanistan and had warned that Sweden might sink Soviet submarines violating its territorial waters.

Sweden has admitted increasing numbers of Middle East refugees in recent years and one theory police were working on was that there could be a link between the slaying and the jailing for life earlier this week of a Kurd accused of political assassination in the city of Uppsala, north of Stockholm.

Palme was born Jan. 30, 1927, into a Baltic noble family but was proud of his involvement with the working class. "I was born into the upper class, but I belong to the labor movement," he once said. "I got there by working for the working class on its own terms, by joining the movement working for freedom, equality and fraternity among people."

Palme, who spoke fluent English, German and French, was educated at Ohio's Kenyon College and at the University of Stockholm.

United Press International added:

World leaders expressed shock at Palme's assassination and said they felt that his legacy would be his contribution to international peace and cooperation. President Reagan issued a statement expressing "profound" sorrow, calling Palme "a man who made compassion the hallmark of Swedish policy." U.N. Secretary General Javier Perez de Cuellar said, "He will be remembered with gratitude and profound respect."