Iranians marched peacefully through Tehran in even greater numbers today in their second successive mass protest. Compared to yesterday, today's marchers took a markedly more political turn directly against the shah and his American backers.

Many prudent Iranians, who refrained from demonstrating Sunday for fear of army and police intervention, joined today and trudged in closely packed formation along the five-mile line of march through the city center.

[Angry youths went on a rampage in the central Iranian city of Isfahan, pulling down four statues of the shah and setting fire to several buildings, Washington Post special correspondent William Branigin reported from Isfahan. Details, Page A16.]

Indicative of the tougher, more out-spoken mood in Tehran were banners and chanted slogans that, apparently in violation of bargain between the opposition and the government, condemned Shah Mohammad Reza Pahlavi by name, a tactic avoided Sunday. The United States, the top foreign influence in Iran also was singled out for abuse.

Banners, sometimes in English, read, "We will kill Iran's dictator," "Death to the American establishment," and "We will destroy Yankee power in Iran."

Graffiti on the Shahyad Monument, the shah's tribute to 2,500 years of Persian history and the end of the line of march, proclaimed: "The shah is a dog chained to the Americans," "Yankee go home," and, perhaps as a countervailing afterthought, "U.S.S.R. go home."

The chants, repeated over and over by the crowd, were even more outspoken.

They said, "Arms for the people," "Wait until we get guns," "This American king should be hanged," "Shah, if you don't get the message, you'll get it from the barrel of a machine gun," and "Death to the shah."

Surveying the crowd, which impartial observers judged was larger than Sunday when nearly a million Iranians marched, a young civil engineer said: "That is a referendum -- and it says the shah must go."

Both days' marches came off without widely predicted violence. The most prominent religious opposition leader, Ayatollah Ruhollah Khomeini, had said from his exile in France that "torrents of blood" would flow. But an accord between the opposition and the government, which led to lifting of a martial-law procession ban, was credited with keeping the protests peaceful.

In Washington, a State Department spokesman said the United States continues to hope the shah "can maintain authority and retain control" of his government, but he refused to say whether U.S. officials believe the worst is over for the monarch.

[Spokesman Hodding Carter made the comments as he expressed the department's satisfaction that weekend demonstrations were carried out without violence in Tehran.]

[In London, meanwhile, Amnesty International accused Iranian authorities of continuing systematic torture of political prisoners, contrary to frequent claims by the shah that torture has been stopped. Associate Press reported.]

With the army withdrawn for both marches along a line protecting the shah's palace and northern Tehran neighborhoods favored by foreigners and the rich, there were obvious fears, however, about what would happen Tuesday.

Would the army move back without problems into what in the last 48 hours has become opposition turf? As an 11 p.m. curfew took effect, some police units and army personnel carriers began creeping in from their peripheral positions around the city.

The demonstration organizers -- principally the Moslem clergy and professional and lay associations as well as the purely political opposition -- distributed leaflets beseeching the public not to demonstrate Tuesday. The leaflets said SAVAK, the secret police, was organizing marches, apparently to cause trouble and discredit the opposition.

Observers noted the overt political overtones of the demonstrations on Ashura, the major event of the Shiite month of mourning called Moharram. Some had expected the day to be more religious in keeping with the 1,298th anniversary of the assassination of Hossein, grandson of the prophet Mohammed and founder of the Shiite sect of Islam, which is Iran's official religion.

Traditionally, Ashura is observed by Iranians beating their chests in mourning until they are bloody as a sign of grief. But Iranians today appeared almost light-hearted and obviously enjoyed the freedom to let off steam and criticize the shah and his government without fear of arrest.

For example, a group of women poked fun at Gen. Gholam Reza Azhari, the military government's prime minister, who recently suggested the opposition was tape recording antigovernment slogans and broadcasting them from rooftops at night to give the impression of greater numbers.

The women chanted "Azhari, you still think it's all done with tapes?"

One middle class professional seemed to sum up the problem posed by the tremendous public participation in the marches against the Shah when he said: "We would settle for the 1906 constitution," which would reduce the shah to the role of constitutional monarch.

"But they," he said, indicating the crowd, "want the end of the monarchy and as you can see they are far more numerous."

Even if the army redeploys without problems throughout the capital observers question government assumptions that the understanding on security for the Ashura holy days means that the opposition has made its first step toward accepting a new civilian government.

At this point no civilian politician of prominence would be likely to brave the public -- and especially Khomeini -- and join such a cabinet.

Privately, opposition politicians say any such Cabinet to be credible would within 24 hours of assuming office have to lift martial law, end the military government, lift censorship and allow freedom of assembly.

That in the present atmosphere probably is a prescription for disaster and the best reason that the shah seems likely to continue with his military government until the next upheaval. If the marchers of the past two days are telling the truth, that one may be armed insurrection.