Indian police shot dead 20 sandalwood smugglers on Tuesday during the biggest operation for years to stamp out trafficking of the rare commodity, law-enforcement authorities said.

Opposition politicians and human rights activists challenged the report, maintaining that those killed in the incident in the southern state of Andhra Pradesh were only unarmed laborers and calling for an investigation.

Andhra Pradesh Police Chief J.V. Ramudu told reporters that a dozen-strong police task force had come under a hail of stones launched by around 100 smugglers who were also brandishing axes, sickles and other sharp-edged weapons.

Police were forced to fire back in self-defense, he said. Ramudu said nine smugglers were killed in the Chitoor district and 11 in a second clash half a mile away.

M. Kantha Rao, deputy inspector general of a special police task force, said that the smugglers attacked police and forestry officials as they were looking for 200 smugglers near the Hindu pilgrimage town of Tirupati.

All 20 were from the adjacent state of Tamil Nadu. Human Rights Forum, a regional advocacy group, demanded an investigation, saying the protesters were unarmed laborers.

“We think it was a fake encounter,” forum general secretary V.S. Krishna said. “Police liquidated them in cold blood and then concocted the story of an exchange of fire.”

Andhra Pradesh is home to stocks of red sandalwood, whose felling, transport and sale is prohibited in India but which is highly prized for furniture in China and Japan.

State authorities created a task force last year comprising police and forest officers to crack down on the smuggling. By the end of 2014, they had arrested more than 500 people.

India's most notorious sandalwood smuggler, Koose Muniswamy Veerappan, was shot dead in a gun battle in 2004, but since then smaller gangs have sprung up. Most of the wood has been smuggled out through northeastern India into Myanmar.

Tamil Nadu Chief Minister O. Panneerselvam wrote to his Andhra Pradesh counterpart asking for a quick investigation.

“There are laws to take action against those cutting and stealing from the forest, but firing and killing is totally unjustified,” said V. Gopalsamy, a leader of the opposition Tamil Nadu-based MDMK party.

Wire services