Makassar on Sulawesi island doled out some tough love with raids at hotels and guest houses on Thursday evening which netted about two dozen unmarried offenders, including a German national.
"We caught the German with his Indonesian partner in a motel and they weren't husband and wife so that's why we arrested them," Iman Hud, head of the local public order office, told AFP.
The unlucky lovers were quickly released after a lecture about the evils of pre-marital sex, but five sex workers caught in the dragnet would be sent to a rehabilitation centre, he added.
"These social illnesses must be prevented. We need to remind the public to uphold our culture and ethics," Hud said.
Valentine's Day is controversial in parts of the majority Muslim nation, with many Islamic clerics and Indonesians criticising its Western roots.
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Still, many others practise a moderate form of Islam and celebrate the day with chocolates and flowers for their loved ones, and displays were set up at malls and cafés in the capital Jakarta.
Back in Makassar, however, authorities were checking to see if shops had complied with an earlier warning not to sell condoms openly and check identification cards to make sure buyers weren't under aged.
Makassar's acting mayor Muhammad Iqbal Samad Suhae insisted the measures were necessary to prevent his city from being paralysed by illegal activities and drug use.
"Festivities like Valentine's Day usually attract youth," he added.
"That's when they are out of control and doing things which violate our norms and traditions, like consuming drugs and engaging in other activities.
"We want to prevent that here."
In Depok city near Jakarta, administrators warned students against any Valentine's romance under threat of unspecified sanctions for violators.
Across the archipelago in conservative Aceh, the only region in Indonesia that imposes Islamic law, a government circular called for residents not to celebrate the romantic day and to report any violations.
The document also told restaurants, cafés and hotels not to provide space for celebrations and asked clerics to deliver speeches on the danger of Valentine's Day.
The latest crackdown comes after the national government last year backed off a bill that would have made pre-marital sex illegal.
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