"There must be a criminal investigation to determine whether the police used excessive force, and whether the killings amount to 'fake encounters,' or staged extrajudicial executions", said Abhirr Vp, of Amnesty International India. "The police are not above the law, and must not be treated like they are."
Those responsible should be brought to justice, according to Amnesty.
"Lethal force should not be used except when strictly unavoidable in order to protect life," Vp said.
A police official, Deputy Inspector General Kantha Rao, declined comment Wednesday, saying an "investigation is still going on."
Police say shooting were in self-defense
The incident in question took place early Tuesday in India's southeastern Andhra Pradesh state.
Forest officials near the town of Tirupati spotted hundreds of smugglers cutting trees for red sandalwood, Rao said Tuesday. The forest officials, who were unarmed, called for police.
"We saw several trees chopped down," Rao said said. "These guys came prepared and were heavily armed."
Rao said that the ensuing gun battle lasted more than three hours and 20 smugglers were shot. Many of them came from the neighboring state of Tamil Nadu, he said.
Amnesty International said the police were claiming self-defense, alleging the suspects attacked them with stone and axes.
"However, no police official was injured," the Amnesty statement said. "Most of the suspected smugglers were shot in the back."
'Violation of human rights'
According to Amnesty, India's National Human Rights Commission has said the incident "involved a serious violation of human rights of the individuals," and that "the opening of firing cannot be justified on the ground of self defense since it resulted in the loss of lives of 20 persons."
The commission has asked senior government and police officials to explain the actions of the police and forest officials within two weeks, Amnesty said.
Police said sandalwood smuggling is common in the area.
Local media are reporting that politicians from Tamil Nadu have lodged a protest against the Andhra Pradesh government, saying the alleged smugglers were murdered in cold blood.