Indonesia's two largest Muslim organizations, claiming nearly one-third of the population as members, today urged the government of President Megawati Sukarnoputri to crack down on Islamic militants suspected of violent activities.

Following the arrest of a radical cleric, Abubakar Baasyir, the two main groups representing mainstream Muslims here have joined the public debate, tagging Islamic extremists as a fringe minority in the world's most populous Muslim nation.

Leaders from Nahdlatul Ulama and Muhammadiyah have rallied behind a pair of government decrees issued over the weekend that make it easier to detain people suspected of involvement in terrorist activities, such as the massive bombing of Bali's nightclub district on Oct. 12, which killed at least 183 people. Some moderate Muslim leaders have urged Megawati to propose even more draconian measures.

"We badly need such regulations to prevent terrorist attacks," said Hazim Muzadi, chairman of the 40-million member Nahdlatul Ulama. "All countries have similar laws."

Some human rights groups have warned that the new anti-terrorist measures could be abused and mark a return to the autocratic practices of former president Suharto, who was ousted in 1998. But moderate Muslim figures said these fears are overstated.

"It's rubbish," said Syafii Maarif, chairman of Muhammadiyah, which has 30 million members. "If those so-called radicals did not do anything wrong, they should not worry."

Nahdlatul Ulama and Muhammadiyah have emerged as a force for stability in Indonesian politics. Their leaders were outspoken in their opposition to an effort last summer by Muslim militants to heighten the role of sharia, or Islamic law, which would have proved highly divisive even though nearly 90 percent of Indonesians classify themselves as Muslim.

Muhammadiyah represents many of the country's more modern Muslims and has a large following in the cities. Nahdlatul Ulama draws its support from a more rural population that tends to follow a form of Islam heavily tempered by other traditional religious beliefs and cultural practices.

Together, they reflect a society that is more moderate in both politics and religious practice than many other Muslim countries. They have long been at odds with some of the more radical Islamic groups in Indonesia, including those associated with Baasyir.

While some government officials warned that Baasyir's arrest could provoke a backlash from Muslim militants, former president Abdurrahman Wahid, a leader of Nahdlatul Ulama, declared his support for the move. "I believe that Baasyir is a terrorist," he said during a recent radio interview.

Police handed Baasyir an arrest warrant Saturday but have yet to question him. He remains under police guard in a hospital in the central Java city of Solo after complaining Friday of heart and respiratory problems.

Baasyir, the founder of an Islamic boarding school in Solo, has been identified by U.S. and Asian intelligence officials as the head of Jemaah Islamiah, a radical network active in several Southeast Asian countries. These officials said the group has links with the al Qaeda terrorist network.

Though Indonesian security officials suspect Jemaah Islamiah members were involved in the Bali attack, Baasyir was not charged in connection with that bombing. Instead, he was arrested for his alleged involvement in a series of church bombings in Jakarta and nine other Indonesian cities on Christmas Eve 2000. Those explosions killed 19 people.

Investigators disclosed today that the main bomb in the Bali attack was made of ammonium nitrate, a compound stockpiled by Jemaah Islamiah, the Associated Press reported.

Baasyir has denied participating in terrorist activities and said that Jemaah Islamiah does not exist.