By Agustinus Beo Da Costa and Kanupriya Kapoor

JAKARTA (Reuters) - Indonesian police on Wednesday said they were investigating whether the controversial leader of a hardline Islamist group insulted the country's secular state ideology, as concern grows over the clout of fundamentalist groups.

National police chief Tito Karnavian has warned against creeping Islamic extremism in the country of 250 million, home to dozens of religious and ethnic groups, and where the vast majority of citizens practice a moderate form of Islam.

Police are questioning Habib Rizieq, head of the hardline Islamic Defenders Front (FPI), a key organizer of recent rallies by hundreds of thousands of Muslims against Jakarta's governor, an ethnic Chinese Christian, who is on trial over accusations that he insulted the Muslim holy book, the Koran.

"We have started a formal investigation...We may name him a suspect soon," said West Java police spokesman Yusri Yunus.

"Unfortunately, democracy is being misused by certain groups to limit other people's freedoms, for instance, by those with fundamentalist or Wahabi thinking who then target minorities," he added, referring to a puritanical strand of Islam originating in Saudi Arabia.

Police in the province of West Java have been questioning Rizieq over claims that he made defamatory comments in 2014 about one of Indonesia's founding fathers, Sukarno, and questioned the legitimacy of the state ideology Pancasila.

If found guilty, he could face up to four years in prison.

Rizieq has denied wrongdoing, according to media. Reuters could not immediately reach him, or his representative, for comment.

Police said the complaint was brought against Rizieq in October last year by Sukarno's daughter, Sukmawati Sukarnoputri.

The cleric and his organization FPI also had their accounts on social network Twitter suspended this week over what the company called violations of its rules. Twitter did not say how the group violated its rules, media reported.

Rizieq and his followers have vowed to press their demand for Jakarta governor Basuki Tjahaja Purnama to be jailed, with another rally set for February 11, just days before the Jakarta governor elections.

"We see what happened with the Twitter accounts as a form of discrimination against the Muslim people," said Novel Bamukmin, head of the FPI's Jakarta chapter, which is calling for a boycott of Twitter.

Late last year the FPI led some of Indonesia's biggest rallies in nearly 20 years targeting the Jakarta governor.

There are concerns Purnama has been unfairly targeted as he runs for reelection next month and that the government has not done enough to protect the rights of religious and ethnic minorities.

(Writing by Kanupriya Kapoor; Editing by Ed Davies)