JAKARTA (Reuters) - Indonesian President Joko Widodo called for calm on Thursday amid simmering religious tension as helicopters dropped police leaflets over the capital, warning residents of the risk of harsh penalties if new rallies led by Islamists turn violent.

Widodo sought this week to reassure investors and show his political coalition is united after over 100,000 Muslims, led by hardline groups, took to the streets on Nov. 4 to call for the ouster of Jakarta's Christian governor, Basuki Tjahaja Purnama, accused of insulting the Koran.

One person was killed and more than 100 were wounded when the protest, the biggest in the city in recent years, briefly turned violent, and police fired tear gas and water cannon.

"I just want to convey one word and that is optimism. Let us not forget that word...even though the political situation is a little heated," Widodo told an investors' forum.

Indonesia has the world's largest Muslim population and is home to Christian and Hindu communities.

Widodo has met with top political, security and religious figures since the Nov. 4 rally, after accusing unidentified "political actors" of inflaming the tension.

The trigger was a comment that Jakarta governor Purnama, the first Christian and ethnic Chinese in the job, made about his opponents' use of the Koran in political campaigning.

Police last week named Purnama a suspect in the blasphemy probe. He faces up to five years in prison if found guilty.

Despite moderate Muslim groups now calling for restraint, Indonesia's police chief has said that some demonstrators may use rallies on Friday to destabilize the government.

One hardline group, the Islamic Defenders Front (FPI), said its members planned to march again on Friday week, though it pledged it would be peaceful.

Police plan to deploy 18,000 officers for any new protests and have used helicopters to drop about 50,000 leaflets in parts of the capital of 10 million, where high-rise apartments sit next to slums.

The leaflets warn residents not to disrupt public order or undertake "subversive" activities, which carry punishments including death or life in prison.

"These are extra security measures and preemptive moves to remind the public not to violate the law," Jakarta police spokesman Awi Setiyono said.

Purnama, who is running for reelection in February, is up against two Muslim rivals. A poll published Thursday showed he has slipped to second place as his popularity declines amid the blasphemy allegations.

Rival Agus Yudhoyono, son of previous president Susilo Bambang Yudhoyono, had taken the lead, the poll showed.

(Reporting by Kanupriya Kapoor and Hidayat Setiaji; Editing by Nick Macfie)