The military arm of Iran's secret Revolutionary Council said today it hopes to spread the Islamic revolution abroad and combat armed opponents of Ayatollah Ruhollah Khomeini's government at home.

The statement by the command headquarters of the "Army of the Guardians of the Islamic Revolution" was greeted with apprehension by Iranian liberals and leftists, who fear the militia may eventually be used to crush dissidents critical of Iran's strict Islamic system.

The communique again raised questions about who is going to have final say in postrevolutionary Iran, the clergymen of Khomeini's Revolutionary Council or the civilian and more liberal government of Prime Minister Mehdi Bazargan. The declaration appeared to contradict a statement by Bazargan only last week that such revolutionary organizations will be dissolved along with the installation of a permanent elected government.

On orders from Khomeini, the Revolutionary Council announced formal establishment of the guards under its direct control yesterday. Some observers linked this with increasingly strident anticommunist statements from the Khomeini camp since the assassination last Tuesday of Ayatollah Morteza Motahari, a key Khomeini aide and Revolutionary Council member.

The announcement said the Council had appointed a command staff whose membership was not revealed. The revolutionary guards were first formed in February after the insurrection that overthrew Shah Mohammad Reza Pahlavi. They have not been as well-organized as Marxist and radical Islamic guerrilla groups, however, and their purpose and command structure were never clearly defined.

The latest declaration appeared to signal an effort to change all that and set up an armed force designed to execute the orders of Khomeini and his mysterious Revolutionary Council. It said the duty of the new army would be to "protect the revolution in Iran and spread it in the world on the basis of genuine Islamic ideology."

It also said the organization would "support just liberation movements of the impoverished masses of the world under the leadership of the revolution and in consultation with the government."

In addition, the force is charged with helping maintain internal security, fight counterrevolutionaries, defend against foreign aggression, carry out the orders of Islamic revolutionary courts and help provide "moral, ideological and military training" for army officers.

The declaration said the guards would "wage armed struggles against those who attempt armed struggles against Islam."

In what interpreted as a thinly veiled warning to Iranian leftistts-blamed by many Khomeini followers for the slaying of Motahari-a senior figure in the ayatollah's entourage said the clergy was the only force restraining Iran's vast Islamic majority from attackin the leftists.

Another religious leader put it even more bluntly. "Just as we destroyed the giants of Western imperialism," he said, "we will destroy communism, Zionism and any other 'ism' which is against Islam."

The pro-Moscow Tudeh Communist Party reacted by denying it had anything to do with the assassination of Motahari and charging that rightists were taking advantage of the incident to stir up anticommunist sentiment.

Although the guards' charter spoke of an international role, political observers here said the force would probably have its hands full trying to enforce its orders in Iran, given the diverse armed groups and the threat of further political assassinations.

It was not immediately clear how large a force the revolutionary guard will be. Tehran newspapers estimated the organization has about 6,000 militiamen. But some officials put the figure at "a few thousand."

In an interview, Sadegh Ghotbzadeh, head of National Iranian Radio and Television and a close Khomeini aide, said the guard has a "very important role because of the lack of reorganization in the armed forces." He said the force is meant to be "supplementary" to the regular Army and is "definitely ideologically oriented."

Ghotbzadeh said the revolutionary guard would be a permanent force under direct authority of the Revolutionary Council and later would be placed under the joint control of the Council and the government once an elected administration is installed.

Contradicting a statement by Bazargan in an earlier interview, Ghotbzadeh said, "After the election the Revolutionary Council should exist but its role should be one of guidance." Bazargan had said the Council would be dissolved and replaced by an elected parliament when power is transferred to a permanent government.

Ghotbzadeh is widely believed to be a member of the Council. In the interview he would neither confirm nor deny this.

He said the militiamen of Khomeini's much-criticized revolutionary committees would be partly absorbed by the revolutionary guard and partly integrated into the country's economy.He confirmed that the committees are to be disbanded, possibly even before the installation of an elected government.