Islamic conservatives in Indonesia, the world's biggest Muslim-populated nation, voiced opposition to Thursday's Valentine's Day celebrations, saying they promote teen "sex parties" and premarital sex, AFP reports.

After the deputy mayor in the conservative city of Depok on Jakarta's outskirts urged parents to stop their children from celebrating, local religious leaders voiced support.

"We don't want to completely ban Valentine's Day but it's not for teenagers," Depok secretary of the country's largest Muslim organisation Nahdlatul Ulama, Raden Salamun Adiningrat, told AFP.

"Teenagers see it as a time to express love and affection and they often end up in sex parties, like in the West."

The Head of the hardline Islamic Defenders Front (FPI) in Depok, Habib Idrus Al Gadhri, declared the day "haram" (forbidden) for Muslims, claiming it is part of the culture of "infidels".

"It's Western culture and immoral so it's ironic Muslims celebrate it," he said.

Despite the calls, many of the country's glitzy malls, hotels and restaurants are offering love-themed banquets and special events.

Around 90 percent of Indonesia's 240 million people are Muslim but the vast majority practise a moderate form of the religion.

Nonetheless, dozens of schoolgirls wearing headscarves in eastern Java's Malang city protested against the day, declaring February 14 "headscarf day".

With plans to protest in several cities, the girls have distributed pamphlets illustrating how men and women should dress modestly, covered from head to toe.

Local media reported protests in the central Java city of Solo on Wednesday, where nearly 500 elementary school students and teachers marched against youths celebrating the day. They carried signs reading "Valentine, Infidel Culture".

Authorities in some cities such as Balikpapan on Borneo reportedly said they would monitor cheap hotels and conduct raids on unmarried young couples.

The Indonesian Council of Ulema (MUI), the country's top Muslim body, has declared Valentine's Day haram several times in recent years.

Islamic organisations such as the Solo MUI chapter and the FPI last week called on Muslims not to take part in Chinese New Year celebrations on the grounds these were linked to Buddhist rituals.