A special U.S. Army envoy was involved in contingency planning for a military takeover in Iran to restore stability after the shah fled, according to a document released here today by Iranian authorities and alleged to be a "top secret" American cable.

The document was made public at the opening of a government-sponsored, four-day conference on U.S. intervention in Iran. A major purpose of the conference is to prove the Islamic revolutionary government's contention that the Pentagone controlled Iran during the reign of the ousted shah, Mohammad Reza Pahlavi.

The purported cable, dated Jan. 22, 1979, was addressed as an "eyes only" message for Gen. Alexander Haig, the commander of U.S. forces in Europe, from his chief deputy, Gen. Robert Huyser, who at the time was reported to have come here to keep the Iranian military in line and forestall a coup.

The text of the purported message made public today, however, suggests that Huyser's mission was just the opposite. It talks twice about planning with Iranian officials for a military takeover and says Huyser was pushing the Iranian government to use the Army to break a series of strikes by opponents of the shah that had paralyzed the country.

National Security Council spokesman Alfred Friendly Jr. said in Washington that "the White House has no comment to make on any material of this sort released or to be released from the conference now under way in Tehran."

[Haig said through a spokesman that he was not in the chain of command between Huyser and Washington at that time. The spokesman said he was not authorized to confirm or deny the authenticity of the cable or to discuss it further.]

[Defense Department spokesman Tom Lambert said the Pentagon had no comment on the allegations.]

It was not made clear where the alleged document came from. Mansour Farhang, the Iranian ambassador to the United Nations, told reporters that it had come from the files of the Iranian Defense Ministry but he offered no explanation how a copy of a top-secret U.S. cable would end up there.

The document, which I examined, appeared to be a computer printout. Militants who seized the U.S. Embassy last Nov. 4 have said in the past that they found classified U.S. documents and computer tapes in the embassy.

The conference that opened today is also aimed at justifying and seizure of the U.S. Embassy here and the holding of 53 American hostages by showing that U.S. diplomats were not entitled to normal diplomatic immunity since they acted as spies and military rulers.

Ayatollah Ruhollah Khomeini, in a message sent to the opening session, charged that "American advisers had direct control over our destiny." Weinglass, one of 10 Americans who defied a U.S. ban on travel to Iran to attend the conference, said of the purported cable from Huyser to Haig, "I think the message is very damning evidence if borne out by the documents, of U.S. interference in Iran's internal affairs."

The Huyser mission to Iran came in a turbulent period after the shah was forced out of the country by a series of violenty demonstrations against him and just before Khomeini returned triumphantly on Feb. 1 to proclaim the Islamic revolutionary government that now rules here.

During the period, the shah left Iran in the hands of his appointed prime minister, Shahpour Bakhtiar, whom the United States regarded as a moderating influence but whom Khomeini refused to accept as head of government.

It appears from the purported message that Huyser was urging the Iranian military to take a more active role in supporting the Bakhtiar government -- including using the Army as strikebreakers.

"The actions I am pressing," Huyser purportedly cabled Haig, "are to break the strike by the use of military in customs, oil and banking. We have made some progress in all three areas but have a long way to go."

"If that fails," the document said, apparently referring to Huyser's alleged use of the military to hold power, "my guidance to them is that we must go to a straight military takeover."

"We are working on this planning on a high priority, 24-hour-per-day basis," he said later in the message. It was not clear in either reference whether a takeover of the government or merely a takeover of the struck oil-fields was meant.

Still later in the document, Huyser purportedly said the military has "a fairly high capability to do the job," apparently referring to the unspecified takeover.

"In fact," the document continued, "we are planning for that option if necessary. The point I want Washington to understand is the military does not have the capability, after doing the initial job, of picking up and running a sophisticated government like they now have established."

In fact, the military proved to be no match for the popular fervor of the Islamic revolution. On Jan. 26, four days after the date on the alleged cable, 100,000 Khomeini backers marched through the streets of Tehran in defiance of a martial law ban. Troops fired and killed at least 15.

From that point on, the power of the Iranian military eroded as base after base turned themselves over to the revolutionary forces.

The purported Huyser message said that if Khomeini returned soon, as he did, "I believe there would be a big upheaval and then things would go to hell in a handbasket."

A phrase in the document that Iranian today interpreted as a U.S. assasination plot on Khomeini said that some factions -- probably leftists -- would like a complete civil war in Iran. "One good way to trigger it," the document said, said, "is to have Khomeini return and be assassinated. Then emotions would take over and I believe the result would be civil war."

The month-long Huyser mission to Iran has been shrouded in controversy and mystery. It had been thought that Iran's military, the best equipped in the area, could keep the shah in power. Late last month, in an interview from his exile palace in Cairo with Washington Post Co. board chairman Katharine Graham, the deposed shah regrettled not using military force to put down demonstrations against him.

In the end, however, the troops either refused or were ordered not to fire on demonstrators.

The conference is being attended by about 400 delegates from nongovernment groups around the world. Members of groups representing themselves as coming from conservative Arab states, such as Saudi Arabia and Bahrain, sat in the conference hall with headdress wrapped around their faces and sunglasses over their eyes so they could not be identified.

The American delegation is headed by former attorney general Ramsey Clark, who has supported the Khomeini government from the start.

With him in the delegation is John Cerassi, a political scientist and former Newsweek writer, who was prosecuted by the Justice Department when Clark was attorney general for traveling to Hanoi during the Vietnam War.

[State Department officials said the Carter administration will "decide in due course" whether to prosecute Clark and the other Americans for defying the U.S. ban on travel to Iran. White House press secretary Jody Powell added that "three are civil and criminal penalities available" to use against the 10.]

Iranian authorities said the conferenct, called by Khomeini to protest the aborted U.S. hostage rescue mission, had nothing to do with the release of the hostages. Clark, however, expressed hope that greater understanding as a result of the meeting will make it easier to obtain the American's release.