PHNOM PENH, NOV. 29 -- An estimated 20,000 Vietnamese troops began heading home from the Cambodian capital and the southern port of Kompong Som today.

The troop withdrawal, the sixth since 1982, precedes a meeting in Paris scheduled for Wednesday between Cambodian resistance leader Prince Norodom Sihanouk and Prime Minister Hun Sen of the Vietnamese-backed government in Phnom Penh. Their session will mark the first time that representatives of the opposing Cambodian factions have met to discuss a solution to the nine-year Cambodian conflict.

Vietnamese officials claim this year's pullout is twice as large as previous withdrawals. But western and Southeast Asian countries opposed to Vietnam's occupation of Cambodia charge that Hanoi is merely rotating troops. Western intelligence sources in Bangkok report that Vietnam brought in 12,000 to 15,000 new troops in recent weeks to fill in for the departing soldiers.

Previous Vietnamese troop withdrawals have taken place in the spring, prior to the monsoon rains, and this year's delay has prompted speculation that Hanoi is facing difficulty building a Cambodian army to replace the departing Vietnamese.

Hanoi approached dozens of nonaligned and western countries to observe this year's withdrawal, but only 18 countries and organizations closely aligned with the Soviet Union and Vietnam accepted the invitation. India, the only non-Soviet bloc country to recognize the Phnom Penh regime, sent a journalist from a state-owned radio network.

The hundreds of departing vehicles were loaded with cooking pots, firewood, spare tires and gasoline containers for the trip to Vietnam. Several dozen Army trucks pulled a variety of old Soviet anti-aircraft guns and American howitzers. Some of the weapons were more than 30 years old. There were no tanks or armored personnel carriers in today's procession from the capital, but Vietnamese sources here claimed some would be withdrawn down the Mekong River to Vietnam.

A half-dozen soldiers interviewed during a farewell ceremony in the western province of Battambang on Thursday said they had served in Cambodia since December 1978, when Vietnam invaded. The invasion ousted the ruling Khmer Rouge regime, which was responsible for killing hundreds of thousands of Cambodians during its four-year rule.

U.S. officials estimated that Vietnam had 125,000 to 140,000 troops in Cambodia before this week's withdrawal, but Vietnamese officials claim that fewer than 100,000 remain. The Vietnamese are fighting an estimated 15,000 to 20,000 guerrillas loyal to the Khmer Rouge and two other noncommunist resistance groups who are united in a coalition government recognized by the United Nations.

Vietnamese troops interviewed in Battambang last week said they had seen little fighting since they destroyed a string of guerrilla bases along the Thai-Cambodian border in 1985. One soldier said the biggest problems they had faced in recent years were mines, malaria, the lack of water during the dry season and homesickness.

Diplomats and aid workers here say they have heard of few guerrilla attacks this year and report that traffic along roads to western Cambodia has picked up in recent months, suggesting that security has improved. A series of small bomb explosions in Phnom Penh this summer ended abruptly after two groups of suspects were arrested.

Vietnam has pledged repeatedly to withdraw all its forces from Cambodia by 1990. But Cambodian Foreign Minister Kong Korm said in a press conference here Friday that "if outside forces take advantage of the withdrawal of Vietnamese volunteer troops and create more instability, then we will discuss with our Vietnamese friends new measures to adopt."