Mobs of angry youths went on a rampage here today, pulling down four statues of the shah and setting fire to a bank and two movie houses.

The rioting broke out at the close of a day of peaceful but undisciplined demonstrations against the rule of Shah Mohammad Reza Pahlavi called on the occasion of Ashura, the holy day of religious mourning for Iran's Shiite Moslems.

Martial law forces, which had kept out of sight since Sunday, allowed the protest marches to proceed without interruption. The troops intervened to stop the later street violence three hours after it exploded.

Machine gun and fifle fire could be heard as several truckloads of soldiers dispersed the mob. There was no immediate word of any casualties, but one person was reported seriously wounded earlier when a crowd carrying clubs and metal bars tried to attack local headquarters of SAVAK, the secret police.

For most of the day, the protesters had the run of the street. The only military presence visible in Iran's second-biggest city until the violence were the helicopters that flew constantly overhead.

Coming after a day of peaceful marches in cities around the country, the mob violence made it clear that there is a hard core opposition that will resist any attempts by politicians to channel its protests.

By contrast, Sunday was a day of peaceful protest marches in this picturesque ancient city on the bank of the Zayandeh River in central Iran.

The marches had the reluctant blessing of martial law authorities, who lifted a ban on all demonstrations including religious processions. Up to 100,000 people, roughly a tenth of the city's population, were estimated to have taken part.

On Monday, several smaller marches started in the morning and lasted into the afternoon. The protesters were less organized and shouted more hard-line slogans, notably "Death to the shah." By late afternoon, mobs of youths began attacking statues in different parts of the city.

At one major square, a crowd of up to 2,000 people gathered and watched as about 100 youths tore down and equestrian statue of the shah with the help of ropes tied to cars.

They toppled flagpoles and set up barricades on a street running along the river between the square and the SAVAK headquarters about 500 yards away.

Security forces stationed in front of the SAVAK compound fired several rounds into the air and shot a few tear gas cannisters toward the square without dispersing the rioters.

The mob scattered a couple of times when truckloads of troops circled the square, but it regrouped after the soldiers passed without opening fire.

The crowd set fire to the statue with gasoline siphoned from cars. cursing the shah, youths hammered the statue with sledgehammers and picks whenever the fire went out.

On the 10-foot-high stone pedestal where the monument had stood, crowds stuck placards bearing a picture of Ayatollah Ruhollah Khomeini, the exiled Shiite Moslem religious leader around whom the opposition has rallied.

As this was happening, other youths took to the streets around the square and exhorted passing motorists to turn on their lights and honk their horns and join in shouting anti-shah slogans.

This they did, some apparently out of fear of having their cars attacked, but most with evident enthusiasm.

At one point a youth began smashing the windows of a closed grocery store owned by an Indian, only to be stopped by others who argued him out of it

They yelled that the shop did not even sell alcohol and prominently displayed a mandatory photograph of Khomeini and the black mourning flag of Ashura.

As the statues smouldered another youth yelled, "cinema" in a call to attack a nearby movie theatre, also closed.

An older man shouted him down, but later other heads prevailed, and the theater's lobby was put to the torch.

The same thing happened on the opposite side of the street to the branch of the Saderat Bank.

That bank had been a frequent target because it was owned by a rich entrepreneur of the Bahai faith, now in jail on corruption charges.

More than Jews or Christians, Bahais are reviled by hardline Shitte Moslems because their faith is considered a heretical offishoot of Islam.

As night fell, thick black smoke clouded the sky over the city, the first in Iran to be put under martial law this year following riots in August. Sporadic explosions resounded into the night as troops fired to scare away crowds with their loud reports.

In a stern, sometimes emotional speech, the martial law administrator here went on local television to announce that the overnight curfew relaxed by three hours last night would be put back to 8 p.m. starting Tuesday.