An international panel of experts hired to advise Hong Kong on the police response to huge pro-democracy protests announced on Wednesday they were quitting, saying the watchdog was not fit for purpose "in a society that values freedoms and rights".

The group's damning conclusion is a blow to Hong Kong's government, which has insisted its Independent Police Complaints Commission is capable of holding the force to account over snowballing claims of brutality.

"We ultimately concluded that a crucial shortfall was evident in the powers, capacity and independent investigative capability of IPCC," the experts said.

Critics have long charged the body lacks adequate powers, is stacked with pro-establishment figures and has been toothless when it comes to keeping the police in line. The watchdog can only handle complaints forwarded by the police themselves and it cannot subpoena documents or compel witnesses to testify.

Such limitations, the expert panel said, do not "begin to meet the standards citizens of Hong Kong would likely require of a police watchdog operating in a society that values freedoms and rights".

Protests have rocked Hong Kong for more than six months, with up to two million people taking to the streets, initially against a now-shelved extradition bill.