ISLAMABAD, PAKISTAN -- A militant new force of pro-Iranian Shiite Moslems has entered Pakistan's turbulent political scene, further complicating this country's efforts to maintain close relations with Arab states of the Persian Gulf and with the United States while also seeking to forge close ties with Tehran.

A crowd estimated at 100,000 gathered in Lahore earlier this month to launch a new Shiite political party that openly advocates closerrelations with Ayatollah Ruhollah Khomeini's Shiite fundamentalist government in neighboring Iran. Party spokesmen warned that no opposition to Khomeini or the Iranian revolution would be tolerated.

Within 48 hours of the Lahore meeting, 13 houses occupied by anti-Khomeini refugees in Karachi and Quetta were attacked with submachine guns, grenades and bazooka-type weapons. At least three supporters of the leftist Iranian opposition group, the People's Mojahedin, were killed and many were injured.

After the attack, 13 men were taken into custody in Quetta and subsequently identified as members of the Iranian Revolutionary Guard. The Karachi attack is also believed to have been carried out by Revolutionary Guard members who slipped into Pakistan, but the head of the new Shiite party says seven members of his group's militant youth wing also were arrested in connection with the attack.

Despite the arrests, the Pakistani government has made no major public protest to Iran and has begun rounding up large numbers of anti-Khomeini Iranians who had settled in the Karachi area.

A high Pakistani official, asked about the government's stance, responded: "Tell me: We are fighting with India, with Afghanistan, with the Soviets; how can we open another front with Iran?"

Other Pakistani strategists point to their country's good relations with pre-Khomeini Iran as having been a stabilizing force in the region and say that Islamabad must continue to try to keep close ties with Tehran even though it now is in a revolutionary fervor.

Previously, Pakistan's own Shiite population has not been a factor in these international political calculations. All that is changing now.

Shiites are estimated to make up between 10 and 15 percent of Pakistan's 100 million people but are concentrated in several key areas: in Baluchistan and the North-West Frontier Province near the Iranian border, near the northern borders with China and India, in Karachi, Pakistan's largest city, and in a politically important region of central Punjab Province.

The majority of Pakistanis are Sunni Moslems and many are followers of a particularly conservative interpretation of the Koran identified with Saudi Arabia. Despite these theological differences, Islamic sectarian splits played little role in Pakistani politics until 1979, when militant Shiism ousted Shah Mohammad Reza Pahlavi in Iran. The Iranian revolution coincided with decisions by President Mohammed Zia ul-Haq to put an Islamic stamp on Pakistani legal, social and economic life.

Close observers of Pakistani politics date the first real show of unified Shiite strength to July 1980, when thousands of Shiites blockaded Islamabad to protest Islamic legislation that they said followed Sunni doctrine and ignored Shiite beliefs. The Army finally was called in to disperse the demonstrators, and Zia has moved carefully on Islamization ever since to keep Shiite sensitivities in mind.

The organization formed for the 1980 demonstration, called the Movement for Implementation of Shiite Jurisprudence, did not disband, however. On the seventh anniversary of the Islamabad demonstrations, it turned itself into a political party at the Lahore meeting.

"Our first priority is to end the dependence on western values in Pakistan. Politics in Pakistan now is dictated from the West and for the West. Western values have been so accepted and ingrained that people think they are part of Islam," said Mohammed Ali Naqvi, a recent medical school graduate and party activist in Lahore.

"With the revolution in Iran, there should be a reflection of these Islamic values in such a large neighboring country."

Naqvi recognizes that his party's task will be difficult, especially with its narrow-sounding, sectarian name, but he says there will be appeals to established parties to start paying more attention to domestic needs and less to defense.

"We now spend more than half our budget on defense. We can't afford that," Naqvi says. "There must be more emphasis on education and then we can raise the consciousness of the people."

To Naqvi and his party colleagues, as in Iran, it is the United States that is suspect, although there are no kind words for the Soviet Union and especially for its policy in Afghanistan.

At the Lahore rally, the Shiite party leader, Arif Hussein, declared, "Shiites would topple the government in Islamabad if it helped the United States to launch any anti-Iran operation from Pakistan."

The clashes in Karachi and Quetta earlier this month were not the first Iranian or Iran-Iraq violence that has spilled over into Pakistan. Bombings in 1985 at the Pan American World Airways office in Karachi, the Austrian trade commissioner's house, at five French-related establishments in Karachi and Lahore and at an office of the Soviet airline Aeroflot were linked by authorities to Iranian groups, according to well-informed sources.

A particularly bloody sequence began late last year when an Iraqi diplomat was killed by a bomb. Two Iraqi Shiites were kidnaped and beheaded in apparent retaliation. Later, an Iranian naval officer in Karachi for training was killed, apparently by an Iranian dissident.

When the People's Mojahedin began circulating news releases about its newly formed Iranian Liberation Army during the past two months, Tehran apparently decided to move against its foes in Pakistan.

With one eye on evolving international politics and one on its domestic scene, Islamabad has been moving to shore up its ties with Tehran. Along with Turkey, Pakistan has sought to keep open ties to Iran throughout the revolutionary upheaval. In Pakistan's case, this has meant walking carefully, since there are more than 30,000 Pakistani troops in Saudi Arabia and Islamabad maintains especially close ties with a number of the Persian Gulf's Arab states and Jordan.

So far, the tightrope act seems to be working. Iran has just asked Pakistan to handle its interests in France now that relations between Tehran and Paris have been severed. Special correspondent Kamran Khan in Lahore contributed to this report.