Iran's left staged a show of strength in defiance of Ayatollah Ruhollah Khomeini today as an estimated 50,000 persons gathered at a rally organized by a Marxist guerrilla group.

Although it demonstrated popular appeal, the rally was less than half the size of one held earlier this month in support of Prime Minister Mehdi Bazargan, who was appointed by Khomeini to head the provisional Islamic revolutionary government. Perhaps more significant than the turnout, observers said, were in the indications that the Iranian revolution is becoming increasingly polarized between left and right.

Speakers representing oil workers, farmers, factory employes and the Marxist Cherikhay Fedaye Khalq, or People's Sacrifice Guerrillas, called for creation of a "people's army," expulsion of foreign workers and nationalization of banks, industries and foreign capital.

They stopped short of openly criticizing Khomeini, the spiritual leader of Iran's revolution who aims to set up an Islamic republic. But the speakers ers made it clear, to the applause of the crowd, that they were dissatisfied with Khomeini's domination of Bazargan's revolutionary government.

"We want unity, not monopoly of one group at the expense of others," a guerrilla speaker said. "Unity means the participation of all groups and classes with all shades of opinion to achieve the common purpose of eliminating imperialism."

The speaker also read what he said was a message of support from Iranian Kurds, stressing that they are not separatists.

However, there were further reports today of secessionist agitation in the Kurdish region of western Iran. The national radio warned Kurds not to attack Iranian military posts near the border with Iraq, saying troops and garrisons had orders to shoot. The radio also appealed to the population to ignore "a counterrevolutionary" call for marches on army barracks in the border regions.

In another report, the radio said an army captain was executed today in connection with an arson fire at a movie theater in the oil city of Abadan last Aug. 19 in which 377 persons were killed. He was identified as Capt. Monir Taheri.

The broadcast described Tahei only as an "antirevolutionary element," without detailing the role he was accused of playing in the fire. He was the ninth Iranian officer to be executed after condemnation by revolutionary Islamic courts set up under the new government.

The crowd at Tehran University, assembled under a drizzling rain, appeared to represent a cross section of Iranian society, although it seemed most heavily weighted with middle-class people and students. Several persons interviewed at random said they were communists. But many others seemed to be Westernized Iranians who fear an Islamic republic will not give them true democracy.

The organizers held the rally on the university campus, a leftist stronghold, after Khomeini banned their march scheduled for yesterday from Fedaye headquarters to the ayatollah's home.

Halfway through the rally, a procession of Khomeini supporters carrying portraits of their leader marched through the crowd to a campus mosque, where they chanted their own slogans. There were no incidents.

A 26-year-old architect, an avowed communist who was attending the rally with his girlfriend, said:

"It's inevitable that in four or five months there will be a fight between the communists and the government. The government is moving toward some sort of fascism."

He said he would be willing to participate in such an armed struggle.

An Iranian liberal who opposes communism said, "We believe the Khomeini people have become arrogant and want to monopolize politics in Iran. We've got to stop it now or it will be too late."

A retired Central Bank employe who said he belongs to the outlawed pro-Moscow Tudeh Communist Party said, "I'm not opposed to Khomeini, but I'm afraid he will be trapped in the bourgeoisie and the struggle of the people will be ruined."

Another participant was a young Iranian naval officer who said he had some reservations about the illdefined concept of a "people's army" but generally supported the idea of having commanders elected by their troops. He said he advocates less reliance on expensive Western arms in favor of strengthening the troops' political and ideological motivation.

A speaker who said he represented Iranian oil workers issued a series of demands including representation on Khomeini's revolutionary council, formation of "true unions," and establishment of workers' committees to supervise oil production, exports and distribution.

He also called for "elimination" of the Western oil consortium that normally produces and exports the bulk of Iran's crude, and "all other capitalist monopolies."

A speaker reportedly representing Iranian farmers called for "farmers" councils," cancellation of debts to Iranian state banks and a variety of rural development programs including "increased facilities for silkworm farmers."