KUALA LUMPUR, Malaysia — Like a vast majority of Muslims in Malaysia, Mohammad Shah was raised according to the Sunni school of Islam. But when he was about 30, he said, he came to believe that Sunni teachings did not answer all of his questions about Islam. He began reading about the Shiite school of thought, the world’s second-largest Islamic branch, and decided that “Sunni was not right for me.”

“I consider myself the new generation of Malaysian Shia,” said Mr. Mohammad, 33. “My father is Sunni, my mother is Sunni. They are aware that I’m practicing a different school of thought. It’s no problem at all.”

But such acceptance does not extend to Malaysia’s religious authorities.

The Constitution guarantees freedom of religion, but when it comes to Islam, the country’s official religion, only the Sunni denomination is permitted. Other forms, including Shiite Islam, are considered deviant and are not allowed to be spread.

Mr. Mohammad was one of 130 Shiites detained by the religious authorities in December as they observed Ashura, the Shiite holy day commemorating the death of the Prophet Muhammad’s grandson Hussein, in their prayer room in an outer suburb of Kuala Lumpur.

There are no official figures on the number of Shiites in Malaysia, but Shiite leaders estimate that there could be as many as 40,000, many of whom practice their faith secretly. (Muslims make up roughly 60 percent of the country’s population of more than 28 million, according to United States government estimates.)

While sectarian divisions are associated more with countries like Iraq and Pakistan, Islamic experts say Malaysia is an example of a Muslim-majority country where the Shiite branch is banned. They say the recent raid reflects the religious authorities’ reluctance to accept diversity within Islam, and was part of the authorities’ continuing efforts to impose a rigid interpretation of the religion.

There had been some earlier arrests of Shiites since 1996, when the National Fatwa Council, the country’s top Islamic body, clarified that Sunni Islam was the official religion. Still, the December raid, on the prayer room occupied by a Shiite group called the Lovers of the Prophet’s Household, was the first in recent years, said the group’s Iranian-trained leader, Kamil Zuhairi bin Abdul Aziz.

Mr. Kamil and the other Shiites who were detained in the raid have been issued summonses to appear at hearings before the Shariah court in March and April to answer charges that they insulted the religious authorities and that they denied, violated or disputed a fatwa of their directives. The offenses are punishable by a fine of up to 3,000 ringgit, about $980, imprisonment for up to two years, or both.

On a recent evening a small group of men and a handful of women with toddlers in tow climbed the three flights of stairs to the prayer room where the raid took place.

A sign atop the building, which is sandwiched between a mechanic’s workshop and a small cafe on a quiet suburban street, reads “House of Knowledge.” A Koranic verse in Arabic marks the entrance.

Inside the prayer room, the flags of Malaysia and the state of Selangor flank a red and black banner bearing the name of Muhammad’s grandson.

As many as 100 Shiites attend prayers led by Mr. Kamil each week, although he said many Malaysian followers worshiped privately. “Most of the Shia are in hiding because of the oppression,” he said.

He said some feared they would be discriminated against when applying for jobs if it were known that they were Shiites, while others were afraid of being detained by the religious authorities.

Mr. Kamil said some Sunni leaders had claimed that Shiites deviated from the true form of Islam and represented a “threat to national security.”

Since the raid, the group has installed a security grill in the stairwell leading to its prayer room, where a black curtain divides the men’s section from the women’s.

But Mr. Kamil and others attending the prayer session recently insisted that they were not afraid to continue practicing their beliefs. “We are not in fear, but we live in difficulty,” he said.

Calling for dialogue with the Sunni majority, Mr. Kamil said that Malaysian Shiites, some of whom are married to Sunnis, want to live in harmony with all other religions.

A statement issued by a spokesman for the federal government said that the Constitution guaranteed religious freedom to all Malaysians, and that the National Fatwa Council was responsible for guiding the practice of Islam in Malaysia.

“In 1996, the National Fatwa Council issued a ruling that Sunni Islam is the official faith of Muslims in Malaysia,” the statement said. “Under this ruling, which is enforced by Islamic affairs departments in each Malaysian state, Shia Muslims are free to practice their faith, but are not permitted to proselytize. It would be inappropriate for the federal government to comment further on this state-based matter.”

The Selangor State Islamic Religious Department, which carried out the raid, did not respond to repeated requests for comment.

Harussani Zakaria, a member of the National Fatwa Council, said allowing different denominations to practice in Malaysia could lead to disputes. “It already happens in some countries,” he said in a telephone interview. “We don’t want that to come here.”

Prof. Greg Barton, acting director of the Center for Islam and the Modern World at Monash University in Melbourne, Australia, said that Malaysia’s religious authorities had adopted a more rigid approach to Islam in recent decades and that space for public discussion of religion had narrowed under the influence of “Saudi Salafism and Egyptian Brotherhood prejudice.”

“The group that speaks formally for Malaysian Islam is a very narrow group who have taken a very puritanical approach,” Professor Barton said. “The religious bureaucracy has become a very meddling bureaucracy. It has a very pernicious impact on religious freedom, not just for non-Muslims but for Muslims as well.”