PHNOM PENH, CAMBODIA — AS Khmer Rouge officials return to the Cambodian capital they once brutalized, San Soeung says memories of their four-year rule are stark.A provincial engineering official, Mr. San recalls the images of those "lost years" when the radical Marxists ruled Cambodia: children wrenched from parents to work, scavenging for cockroaches, toads, anything to ward off starvation, and rows of empty houses "where all that was left of the people who died was their clothing blowing in the wind." And yet, San, who lost his immediate family, has not told his five children of the suffering in the 1970s and is unsure if they know or understand. "They were difficult years," he says. "We did what we had to do." Under a United Nations peace plan to end the nearly 13-year civil war, the first officials of the radical Marxist Khmer Rouge arrived Sunday in the city they emptied at gunpoint in 1975 and almost four years later fled ahead of the invading Vietnamese. Adamantly unrepentant for causing the death of more than 1 million people, the Khmer Rouge join three rival factions on an interim Cambodian authority headed by the former monarch Prince Norodom Sihanouk. This Supreme National Council will oversee Cambodia with UN administrators and peacekeeping troops until a national election expected by early 1993. The disturbing presence of the Khmer Rouge, blamed for killing one-sixth of Cambodia's people through execution, famine, disease, and overwork in its quest for a communist agrarian revolution, is the first step in a carefully orchestrated drive for power over the next several years, analysts say. It is not believed that the Khmer Rouge guerrillas have abandoned their ambition for reconstructing Cambodia as a perfect socialist state. And they continue to use their military might in some areas to recruit soldiers and village support. Their political appeal is based on a xenophobic call to Khmer nationalism, condemnation of the corruption of the Phnom Penh regime, and censure of the deepening chasm between poor peasants and the urban rich. Some guerrilla officials have exchanged their trademark black pajamas for suits to move in international circles and Phnom Penh's corridors of power. In the background stands Pol Pot, the shadowy 63-year-old supremo whom the Khmer Rouge insist has retired. Living in the twilight zone of the secretive communist organization, he is not known to have been seen in public for more than a decade but nevertheless remains firmly in charge, Cambodian and Western analysts say. "He is the most effective Cambodian of all," says a Western diplomat. "He is the best organizer of the Khmer. He knows the Khmer." The major point of contention in post-war Cambodia, the Khmer Rouge have stirred alarm at home and abroad. The United States and other Western countries, whose support of the three-party Cambodian resistance coalition during the civil war helped legitimize and rebuild the Khmer Rouge, are pressuring China, the guerrillas' main patron in the past, to keep Pol Pot in the backgr ound. Demands to try Pol Pot and other leaders, once downplayed in the interest of striking a peace accord, are reemerging. "It's unbelievable what is happening here. They are returning 12 years after what they've done with the blessing of the international community," says the head of a major international aid agency. FOLLOWING his homecoming last week after more than a decade in exile, Prince Sihanouk has forged a political alliance with Phnom Penh Prime Minister Hun Sen and has insisted that the Khmer Rouge is "increasingly isolated." Sihanouk, who was once kept under house arrest by the Khmer Rouge, is trying to distance himself from his past association in the resistance alliance, observers say. However, despite the horrific past of the Khmer Rouge in Cambodia, it remains the most coherent force in the country, analysts say. Following a plan set down in a secret Pol Pot speech in 1988, they have made inroads into the middle ranks of the bureaucracy, the Phnom Penh military, and the ranks of Phnom Penh's newly reconstructed communists, many of whom are former Khmer Rouge, diplomats say. While hoping to elect several members to a new parliament, the Khmer Rouge also hope to build a support network in villages. Provincial officials say that while in the past the Khmer Rouge forced villagers to fight or carry weapons, in rural areas they now detain people only to deliver political campaign talks. In several provinces near Phnom Penh, political posters for Khieu Samphan, the group's ideologue and chief diplomatic and political representative, are starting to appear, international aid workers say. "Khieu Samphan has some popularity," says a Cambodian analyst. "He is a nationalist, and he is not corrupt." In various Khmer Rouge strongholds, roads, dams, reservoirs, hospitals, and even schools are being built to entice refugees and fortify their political grip, observers say. That approach, in contrast to the brutality of the past, is reportedly creating some rifts within the tightly knit and disciplined guerrilla organization, Western observers say. Yet even as the Khmer Rouge builds popular support in the countryside, educated and urban Cambodians, who suffered the most under the Khmer Rouge, have yet to confront the future threat, some international aid workers say. "There is something genuinely Khmer in this whole Khmer Rouge experience," says a Western aid official. "This is a people not yet ready to face their own past."