Three bombs exploded in a crowded shopping arcade today, killing six persons and injuring about 100, the state radio said. It was the worst such violence against Iran's Islamic government since it ousted the shah 18 months ago.

The blast occurred about 10 a.m. as more than a thousand shoppers, mainly women, were in the downtown mall, which contains about 400 shops.

The bombing was the latest in a series of violent incidents here and abroad that have markedly increased tension in the country as President Abol Hassan Bani-Sadr prepares to nominate a prime minister to head a parliamentary government. The parliament has been given the responsibility to determine the fate of the 52 American hostages; but no early decision is expected.

The three bombs exploded in the underground parking garage of the four-story mall and damaged 68 of the 76 ground-floor stores, many of them heavily. Eight of the injured persons were in serrious condition, the radio said.

The Iranian news media, meanwhile, gave sparse coverage to the killing in Bethesda, Md., of Ali Tabatabai, a former Iranian Embassy official who led a group opposed to the Islamic Republic.

Unlike an unsuccessful assassination attempt on the shah's last prime minister, Shahpour Bakhtiar, in Paris last week, the killing of Tabatabai has not been claimed by any groups in Iran.

A mysterous Iranian group called the Guardians of Islam has claimed responsibility for the Paris attack, but Foreign Minister Sadegh Ghotbzadeh said Iran was not involved.

Ghotbzadeh has come under fire from militants for the denial.

The possibility has been raised that at least some elements in Iran with government backing are trying to wipe out opponents overseas as Libya has been seeking to do recently.

Jalaleddin Farsi, the candidate for prime minister of the Islamic Republican Party, said yesterday that he had advance notification of the Guardians' attack on Bakhtiar, "and I approved of their plan without any reservation," "without any exceptions."

[Ayatollah Mohammed Beheshti, a member of the Revolutionary Council and chairman of the majority Islamic Republican Party, said Wednesday that Moslem law allows for "the formation of an Islamic group to destroy the leaders of blasphemy." Quoted on Tehran Radio monitored in London, he added, however, that such a plan would have to be "studied in detail" by Islamic leaders, the Associated Press reported.]

The government has said that an alleged coup attempt that was broken up two weeks ago intended to put Bakhtiar back in power. More than 300 persons have been arrested, mainly military men, and five have already been executed after a one-day secret trial.

[Tehran Radio reported that 20 more alleged plotters were executed Thursday morning.]

The government maintains that the plot was mounted with help from the United States, Israel and Iraq, but has presented no documentary evidence yet that there was an actual coup attempt.

Nevertheless, Ayatollay Ruhollah Khomeini, the country's supreme religious and political authority, has ordered that those involved be executed "without any exceptions."

Today's bombings took on more potential significance in such a heated atmosphere.

Police did not announce any arrests although an annonymous phone caller told a newspaper that the bombing was carried out by a militant anticlerical group called Forqan, which the government claimed to have crushed in January after arresting about 50 members and executing some.

Some diplomats said a more likely possibility was that leftists were responsible. The Iranian left has come under increasing pressure from the government, causing some parties to go underground.

There was also the possibility that the bombing could have been carried out by religious fanatics demanding that women switch from Western dress to more modest, covered Islamic outfits. Most of the shops in the mall sell Western dresses, but many of the women who shop there wear the traditional head-to-toe chador over such clothing.

As is usual in Iran these days, where an anti-American plot is seen behind every action, some shop owners said they felt certain the United States was indirectly involved.

Three months ago, three persons were killed in a downtown building. No arrests were made.

Meanwhile, Bandi-Sadr delayed once more in announcing to parliament his choice for premier, as the names of a variety of candidates continued to circulate in Tehran.It was reported that the decision may be delayed until Saturday at the earliest.