Vietnamese-backed Cambodian insurgents quickly established a provisional government in Phnom Penh yesterday, one day after capturing the Cambodian capital, while Vietnamese and rebel troops secured their control over most of the country.

The formation of the People's Revolutionary Committee of Cambodia was announced over Radio Hanoi and coincided with a demand by the new government that the United Nations Security Council abandon its plans to debate the Vietnamese-Cambodian conflict as requested last week by the now-deposed government of Prime Minister Pol Pot.

Heng Samrin, a little-known figure who has headed the rebel front since it was formed a month ago, was named president of the new provisional government yesterday, according to Radio Hanoi.

The request to the United Nations, forwarded by the Vietnamese delegation, said that the "Pol Pot clique has been overthrown" and therefore has ceased to exist. A debate on the Cambodian "situation," the cable continued, would be a "flagrant intervention in the internal affairs" of the Cambodian people.

Officials here and in Thailand believe that the Vietnamese and Cambodian insurgent troops now control most of Cambodia. They say, however, that troops loyal to Pol Pot are believed to be mounting heavy but scattered resistence in the western provinces.

Although these pockets of resistance remain, they do not appear to be coordinated from a central guerrilla headquarters, analysts said. Vietnamese and rebel troops, supported by continued bombing, were reported to be concentrating their fighting in those western provinces.

Prince Norodom Sihanouk plans to arrive at the United Nations today to present the case of the Pol Pot government, which was deposed by the lightning two-week offensive by Vietnamese troops and their Cambodian allies.

Some 700 foreign officials trapped in Phnom Penh during the surprise attack arrived in Thailand yesterday after a two-day road journey from the Capital to the Thai-border town of Aranyaprathet, according to news agency reports from Thailand.

These foreign officials, over 600 of whom are Chinese, confirmed the fall of Phnom Penh and the flight of the Pol Pot government.

Neither Pol Pot nor any other major figure in the government has been heard from since the fall.

Earlier, Chinese officials in Peking had said that their embassy staff would remain in Cambodia to support the guerrilla struggle Pol Pot pledged he would wage should he be forced to abandon Phnom Penh. But the Chinese ambassador to Cambodia was one of the foreign officials who crossed into Thailand yesterday.

Diplomats from Burma, Egypt, Romania, North Korea and Yugoslavia made up the remainder of the group.

As many as 20,000 Chinese technicians, advisers and officials may have been caught inside Cambodia by the war. On Sunday, the Front warned all foreigners that they would be "duly punished" should they refuse to surrender to their side.

Heng Samrin, in earlier Front Broadcasts, has been described as a former commander in the Pol Pot army who defected last year.

In addition to Heng Samrin, seven vice-presidents were named yesterday. They were all Cambodians unknown to most Indochina watchers.

One of these deputies, Chea Sim, who is in charge of interior affairs for the new provisional government, had been mentioned in earlier Front broadcasts.

Purges of "pro-Vietnamese" cadre in the Pol Pot government and army had been a regular occurrence since open feuding began between Cambodia and Vietnam in 1977. It is presumed that the Front is composed of some of these Cambodian political and military figures who fled the country in fear of their lives.

Little else is known of these new leaders -- not even the size of the Front -- other than their promises to return to Cambodia the freedoms and traditions and some of the institutions abolished under Pol Pot.

At the United Nations, China was leading the lobbying effort to insure that the Cambodian war remains on the Security Council agenda. The Soviet Union was countering the Chinese effort, pointing out that the Pol Pot government no longer exists and that Sihanouk, therefore, speaks for no nation.

When he arrives today, Sihanouk is scheduled to check into the elegant Waldorf Astoria, far more expensive quarters than the hotels where the previous Pol Pot diplomats resided. He will also be facing a seemingly perennial question at the United Nations over which delegation truly represents Cambodia.