Ayatollah Ruhollah Khomeini said today he was willing to start wity a clean slate with the United States after the shah of Iran's downfall, despite Washington's staunch support for the foundering monarch.

The exiled Moslem religious leader, who has come to symbolize Iranian opposition to Shah Mohammad Reza Pahlavi, made clear in a written question-and-answer interview that future relations depended on an end to what he denounced as U.S. interference in Iranhs domestic affairs.

"Our future relations with the United States will depend entirely on the American government," he said. "If the United States government stops interfering in our affairs and respects our nation, we will deal with it accordingly." Sources close to Khomeini stressed that this was the first time he had offered such a diplomatic olive branch to the United States. By extension he was making a similar gesture to Britain, which also has repeatedly supported the shah in public, and to other Western nations.

The new moderation reflected, according to the insiders, Khomeinihs growing conviction that it is now only a matrer of weeks, perhaps days, before the shah would be overthrown and the ayatollah returned to Iran after a 15-year exile.

However, Khomeini stressed that once the shah fell Iran would cesase selling oil to South Africa. Because of its apatheid policy, and to Israel, which he charged with training agents of SAVAK, the secret police, and "participationg in torturing our militiants."

Indicative of the mood in Khoimeini's entourage were reports that Khomeini will soon announced detailed programs dealing with Iranhs political social and economic future.

Khomeini, 78, is said to have readed a list of men for a future Cabinet of the Islamic republic he foresees. "We'll not follow the model of any other country," he said.

That was taken as an allusion to his intention to avoid what some see as excesses of other Islamic regimes in Saudi Arabia, Libya or Pakistan.

Included in the Cabinet would be Western-educated men, a top aide said, but not members of Iranhs aristocracy such as National Front leader Karim Sanjoabi or Shahpur Bakhtiar.

Khomeini, according to insiders, foresees a Western European parliamentary model for Iran, With special stress on open government after so many years of the shah's dictatorial one-man rule.

"You Americans should understand that afteer Vietnam and Watergate," the aide said.

So concerned is Khomeini's entourage about the potential flight of talented Iranians after the shah's downfall that a television broadcast in Persian has been propared in hopes of reassuring the middle class that only corrupt officials will be punished.

"No one but Khomeini can calm them," an aide said, reflecting the ayatollah's enormous prestige inside Iran and the growing awarenss in his entourage that the revolution he has come to symbolize may be difficult to control even after the shah departs.

Khomeini, in his answers, sought to calm Washington's fears about the future, saying, "It would be a mistake for the American government to fear the shahhs deparrure." He denied rumors that he had been in touch with American officials and insisted, "If anyone claims other wise, he is a liar."

Although Khomeini is opposed to any new arms purchases, a multi-billion-dollar source of income for the United States, his aides said that Iran after the shah would not become a subversive force in the Persian Gulf undermining Saudi Arabia or other oil-based regimes, or on any way cooperate with the communists.

Khomeini reiterated his anger at critics who 'have tried to sully our reputation" by tarring him with a communist brush.

Stressing his basic anticommunism, Khomeini denounced "the Soviet Union and its satellite countries, China and its friends who have been hostile to our movement from the beginning just like the United States and its allies. We have never been in the hands of any regime and never will be."

Asked if he thought fellow Shite Moslems in Bahrain, Iraq, Pakistan and elsewhere would follow his lead in contesting temporal power in their countries, Khomeini said, "The chocie of a people depends on themselves. I am not a prophet. But we have no intention of interfering in any other country's internal affairs."