Indonesian police beat protesters during last month's deadly rioting in Jakarta, Amnesty International said, as it called for an independent probe into "grave human rights violations", including the killing of demonstrators.
In May, peaceful protests against president Joko Widodo's re-election victory in the world's third-biggest democracy erupted into street battles between police and rioters in central Jakarta.
At least nine demonstrators were killed and hundreds more injured in the city's worst violence in years.
Police have been under the spotlight after online videos surfaced that appeared to show a group of officers viciously beating a protester as he lay on the ground.
"Amnesty International found that at least four other men were beaten by the police during that incident," the rights group said in the letter dated Tuesday.
"The evidence we have obtained indicates that the police carried out torture or other ill-treatment against persons in their custody in other areas of Jakarta," it added.
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Amnesty also said it had received "credible reports" that paramilitary police were involved in the deaths of 10 demonstrators including one at a protest in Pontianak on Borneo island.
Usman Hamid, Amnesty Indonesia's executive director, told AFP on Wednesday that the rights group was not directly accusing police of killing demonstrators, but said the reports should be probed by an independent body.
In response, National police spokesman Dedi Prasetyo said police were conducting their own investigation into the beating allegations and protester deaths.
"Let's wait for the results of the probe," he added.
Questions have swirled about how the demonstrators -- including a high school student -- died in the chaos.
Police insisted they did not shoot live rounds, but instead used rubber bullets, water cannons and tear gas to push back the crowds.
Some of the dead were reported to have gunshot wounds.
On Thursday, Indonesia's Constitutional Court is expected to rule on claims by Widodo's defeated challenger, ex-general Prabowo Subianto, that he lost the election because of voter fraud and widespread cheating -- allegations that sparked the riots.