A trio of canoodling couples were publicly flogged in Indonesia's Aceh province on Thursday, charged with breaking local Islamic law that outlaws public displays of affection.

Despite international condemnation, whipping is a common punishment for a range of offenses in the deeply conservative region on Sumatra island, including gambling, drinking alcohol, and having gay or pre-marital sex.

Aceh is the only region in the world's biggest Muslim-majority country that imposes Islamic law.

On Thursday, a masked sharia officer rained down between 20 and 22 strokes from a rattan cane each on three men and three women caught behaving amorously. They had all served several months in prison.

One woman winced and fell over from the painful punishment, which was carried out in front of a mosque in the provincial capital Banda Aceh. Dozens of onlookers, including some visiting Malaysian students, watched the spectacle.

"I'm nervous because this is the first time I've seen something like this," said visitor Muhammad Rushdy. "But it can serve as an important lesson for us from other countries to comply with local regulations."

Banda Aceh's mayor Aminullah Usman repeated his long-held claim that the punishment is a deterrent.

"But we keep telling visitors not to be afraid to visit Banda Aceh... because you won't be flogged if you don't break the law," he said.