The withdrawal of Vietnamese forces from Cambodia has conjured fears of a comeback by the dreaded Khmer Rouge, one of the most brutal regimes of modern times.

Although it has been nearly a decade since the Khmer Rouge were toppled by the Vietnamese invasion, there is a fear in southeast Asia of a reprise of their 1975-78 reign of terror in Cambodia. According to some estimates, up to 3 million people died."One has to be either naive or an idiot to believe that the Khmer Rouge in the '80s can be different from the Khmer Rouge of the '70s," Prince Norodom Sihanouk, Cambodia's former leader, said recently.

Having been both their prisoner and their ally, few outsiders know the Khmer Rouge as well as Sihanouk does.

Similar apprehension is being voiced in the now bustling capital of Phnom Penh, whose citizens died by the tens of thousands in a forced march into the countryside during the Khmer Rouge rule.

And, somewhat ironically, alarms are being sounded in countries that in recent years have called for the withdrawal of the Vietnamese and tried with considerable success to brand Hanoi an international pariah because of its late 1978 invasion of Cambodia.

"We cannot preclude the possibility that a Vietnamese withdrawal might create conditions in which the Khmer Rouge could return to power," said U.S. Rep. Stephen Solarz, D-N.Y. and chairman of the House Foreign Affairs subcommittee on Asia.

Solarz recently described a Khmer Rouge comeback as "morally unthinkable and politically unacceptable" and called for an international conference to face this dilemma.

There have been other proposals, such as holding internationally supervised elections, disarming warring parties, and creating a United Nations peacekeeping force to prevent Khmer Rouge domination of a post-war Cambodia.

"I would like people to know who really is the obstruction to peace in Cambodia: the Vietnamese or the Khmer Rouge," said former Thai prime minister and elder statesman Kukrit Pramoj.

Thailand has been a vocal opponent of Hanoi and allows Chinese arms to flow through its territory to the Khmer Rouge.

Such concern was set off by Hanoi's partial withdrawal Thursday of some of its 50,000 troops. Vietnam, apparently with some Soviet urging, has promised to pull out all its remaining forces by 1990 and said it would enter talks to find a political solution to the protracted conflict.