Pro-Vietnamese Cambodian rebels marched into Phnom Penh yesterday, overthrew the Pol Pot government and gained control over most of Cambodia, according to a rebel broadcast on Radio Hanoi.

A few hours earlier, Radio Phnom Penh had failed to broacast its scheduled programs, the only indication from within the country that the rebels have taken power.

If the Vietnamese-backed Cambodian National United Front for National Salvation is in control of most of Cambodia, it would mean initial success for one of the swiftest and most stunning military takeovers in recent history.

On Dec. 25 the Vietnamese and insurgents launched the offensive that brought them to Phnom Penh in little more than two weeks. U.S. analysts asserted that most of the fighting and the intensive bombing campaign was waged by the Vietnamese, who first fought the Cambodian Communists a year ago in a border dispute.

Last week, the Cambodian government said it would fight to the end and that loss of its capital and part of its territory would not end its struggle against the insurgents.They promised to move into the jungle and wage a protracted guerrilla war should they be forced to abandon Phnom Penh.

Apparently the Vietnamese and Cambodian rebels marched into Phnom Penh from all sides after light bombing of the capital, according to intelligence reports.

There were no confirmed reports of the whereabouts of Pol Pot, the prime minister, or Leng Sary, the foreign minister, of the Cambodian Communist government. Diplomatic sources in Bangkok, however, said a Chinese plance landed in the capital before the reported fall and was able to take off and return to Peking. These sources also said the Siem Reap airport in northwest Cambodia may still be in government hands.

About 20,000 Chinese advisers were believed to be in the country aiding the Pol Pot government and their fate is not know. North Koreans were also stationed in Cambodia as advisers.

[In Peking, former Cambodian ruler Norodom Sihanouk today called on the United Nations to create a military force to throw out what he called the Vietnamese-backed aggressors from the country or else expel Vietnam from the world body, Reuter reported. Sihanouk is expected to leave for New York tomorrow, where he will present Cambodia's case before the U.N. Security Council. in reply to questions, however, Sihanouk said he did not expect the United Nations to do anything useful for Cambodia.]

The United States citicized the Vietnamese yesterday and asked that all their troops be withdrawn from Cambodia. In Peking, Reuter quoted a Chinese government spokesman as saying, "The fall of Phnom Penh does not mean the end of the war but the beginning."

After announcing its march into Phnom Penh and the collapse of the "dictatorial, militarist domination" of the Pol Pot government, the rebel front called on all Cambodians to support their forces in the final battles to gain total control the country. The call was broadcast by Radio Hanoi and monitored by U.S. officials.

The Front claimed yesterday to be in control of the southern provinces of Cambodia, including the port city of Kampong Som and most of the northwest. Earlier in the week, the Vietnamese and the insurgents took over the northeast and the southeast.

Since its formation last month, the Front has promised to return to Cambodia most of the traditional practices abolished during the Pol Pot government, particularly the Buddhist religion.

In their major program, released Dec. 3 when the Front was announced on Radio Hanoi, the rebels said they would permit schools, Buddhist pagodas and monks, the markets, basic democratic freedoms, cities and money -- all abolished under the Pol Pot government. They also promised that families could be reunited.

This program is a direct rebuttal of the harsh government of Pol Pot, who led what has been called one of the most radical and bloodiest revolutions of this century. During his more than three years in power, hundreds of thousands of Cambodians died from malnutrition, disease or summary executions, according to refugee reports.

Vietnam has not taken credit for this lighting offensive, allowing the month-old Front to claim all the victories through dispatches by its news agency broadcast over Radio Hanoi.

Yet, Western and Asian officials estimate that Vietnam sent about 100,000 troops into Cambodia and that Vietnamese pilots flew the juets that conducted an intensive bombing campaign.

The insurgents are said to have had no more than 20,000 troops involved in the fighting.

During the two-week offensive, Cambodian government forces apparently fought fiercely against the invading armies but were quickly isolated from their command headquarters and pounded down by the superior Vietnamese artillery and air power.

The Cambodian people, however, offered no resistance, according to intelligence reports, and in some instances may have aided the Vietnamese and the Front forces.

Attacking first in the northeast, the Vietnamese and Front forces pushed toward the Mekong River and then spread the offensive quickly through the southeast. Yesterday they had cut all the supply lines and roads into Phnom Penh and apparently marched into the city without major resistance from the Cambodian Army, intelligence sources said.

With the installation of the Front in Phnom Penh, the Vietnamese have become the dominatnt influence throughout Indochina, which includes Laos, where 40,000 Vietnamese troops are stationed, as well as Cambodia and Vietnam.

Long-held fears of Vietnamese goals in Southeast Asia were raised throughout the region yesterday, according to reports reaching Washington. Thailand, which is bordered by both Cambodia and Laos, put its troops on full military alert.

Vietnamese plans for an Indochina federation were one of many issues that had divided Cambodia and Vietnam since the two Communist powers won control of their countries in the spring of 1975.

During civil wars they fought against U.S.-backed governments, the Cambodian and Vietnamese Communists were allies, albeit wary ones, and there were few signs of the profound disagreements which led to this year's offensive.

In 1976, the two governments failed to come to an agreement on their common border and by the end of 1977 Cambodia broke off relations with Vietnam. The two became embroiled in a border clash in January 1978 and again last summer.

At issue, however, were deep ideological divisions as well as territorial claims. Vietnam chose to develop along a Soviet model after the war, encouraging aid and trade with other nations.

The Hanoi governmetn also drifted away from China, which gave considerable aid to Vietnam throughout its war, and last fall signed a friendship treaty with the Soviet Union.

The Cambodian communists did not receive the support of Moscow during the Indochina war and as soon as they came to power they allied themselves with Peking.

The Combodians enforced an extreme revolution using forced evacuation of the cities and intense, backbreaking labor in the countryside to achieve a goal of self-sufficiency. It was a primitive and radical version of a Maoist revolution.

But beyond these modern problems the Cambodians and Vietnamese have a long history of enmity. It was a deep-rooted fear of the Vietnamese that led Cambodia to refuse to join an Indochinese federation after the war.