Indonesian police have named a controversial Islamist leader who helped organise protests against Jakarta’s Christian governor as a suspect in a defamation case, as authorities move to rein in the influence of hardliners.

Rizieq Shihab, the head of the Islamic Defenders Front (FPI), was accused on Monday of defamation over comments he made about Indonesia’s founding president, Sukarno, and the state ideology.

Analysts said the move was aimed at silencing a hardline fringe which they warn has grown influential by leading opposition to Jakarta’s governor, Basuki Tjahaja Purnama, over allegations he insulted the Qur’an.

Purnama is standing trial for blasphemy over the comments, in a case that has raised concerns about religious intolerance in the world’s most populous Muslim-majority country.

Announcing the move against the FPI leader, West Java provincial police spokesman Yusri Yunus said: “We have enough evidence, and we have named Rizieq Shihab a suspect.” Naming someone a suspect is a step in the Indonesian legal system which means authorities believe they have enough evidence to consider filing charges.

Shihab, who is not being held in custody, could face a maximum jail term of four years and nine months if found guilty.

Sukarno led the struggle for independence and was Indonesia’s first president from 1945, while the state ideology, Pancasila, are the five founding principles of the country, including belief in one God, and the unity of Indonesia.

The defamation allegedly took place during a speech by Shihab in West Java province. Police did not detail exactly what was said.

The FPI, which helped organise a series of mass protests against Purnama, was long regarded as marginal and not taken seriously but the recent demonstrations have catapulted it to national prominence.

Shihab has been subject to numerous attacks and investigations in recent weeks as forces opposed to the FPI move to silence them. Tobias Basuki, an analyst from Jakarta think-tank the Centre for Strategic and International Studies, said the moves against the FPI were “to silence the radicals”.