Shiite Moslems postponed a scheduled mass demonstration here today after the Pakistan government, bowing to mounting Shiite protests, postponed imposition of the Sunni brand of Islamic law.

The government action followed a Shiite demonstration in the provincial capital of Quetta two weeks ago in which at least 25 persons, including nine policemen, were killed and the Army was forced to step in to restore order.

Another demonstration is scheduled for Saturday in the Pakistani capital of Islamabad, as Shiite Moslems appear to be intensifying their pressure on the government of President Mohammed Zia ul-Haq to block the imposition of Sunni Islamic law, which the Shiites consider to be alien. Sunni Moslems form the dominant Islamic sect in Pakistan.

Protests have flared sporadically since Zia announced his intention soon after taking over in a coup eight years ago to replace western-oriented laws with an Islamic code of justice and social order.

Progress has been slow, however, with Shiites resisting the imposition of laws arising from the Sunni interpretation of the Koran.

Both Sunnis and Shiites believe in Mohammed, the Koran, the sayings of Mohammed and the authority of Islamic law. The main difference is the dispute over the designation of Mohammed's successors.

Even though Zia no longer backs an Islamic law that would have required Shiites to pay a charity tax to the government, or amputation of the right hand of convicted thieves. Shiite Moslems here said they face continuing legal discrimination. In the Sunni interpretation, the hand is amputated at the wrist, while Shiite law calls for the hand to be cut at the knuckles.

Faced with the recent Shiite protests, Zia and Prime Minister Mohammed Khan Junejo appointed a 16-member committee yesterday to come up with a set of Islamic laws that "ensure sectarian harmony and national cohesion."

The committee is charged with examining the causes of the unrest, Religion Minister Maqbool Ahmed Khan said.

Shiites here said the formation of the committee was the first admission by the government that its campaign to install Islamic laws has tended to divide rather than unify the country. Although Sunnis form the dominant Islamic sect, it is unclear how large their majority is. Shiites claim to make up as much as 30 percent of Pakistan's 97.3 million inhabitants. Sunnis, however, say that Shiites are only 5 percent of the population.

While the internal Shiite rivalry may have contributed to the current protests, the wide differences between the Shiite and Sunni interpretations of Islamic laws have sparked violent demonstrations here in recent years.

Shiites are concentrated in this teeming city, where crowds are easily ignited by demonstrations. Moreover, they have received help from the government of Ayatollah Ruhollah Khomeini of Iran, and where Shiites form 90 percent of the population.