Palu, Central Sulawesi – Indonesian police opened fire and killed five people when an angry mob attacked their station with firebombs in a protest over the death of a man in custody, police said on Wednesday.

More than 20 people, including several police officers, were injured during the violent clash late on Tuesday in the town of Buol, Central Sulawesi province, local deputy police chief Dewa Parsana said.

"A big number of residents encircled the police station and threw Molotov bombs," he said. "We had no choice but to open fire towards the protesters because they were very angry and were burning motorcycles parked outside... Five people were killed by the shooting."

Anger over the death Monday of a local man in custody at the station triggered the riot, he said.

Around 300 police reinforcements were sent to the area on Wednesday, the state-run Antara news agency reported.

Thousands of people reportedly attacked the station but police said they could not confirm the numbers.

Central Sulawesi police chief Amin Saleh told Antara he deplored the shooting. "We are checking the firing procedures," he said. Antara did not report if police were investigating the death of the Kasmir in custody.

Indonesia police are often criticized for abusing the human rights of people in custody, including torture.

Police and the Indonesian Military (TNI) have sent reinforcements to the area. (AFP/Antara/Jakarta Globe)