MANILA, Oct 7 (Reuters) - A Philippine court acquitted 19 police officers of the killing during a 2016 prison shootout of a town mayor who had been detained for alleged involvement in illegal drugs, a verdict that dismayed human rights advocates. The death of Albuera Mayor Rolando Espinosa in his cell in Leyte province alarmed activists who have accused the government of turning a blind eye to extra-judicial executions and cover-ups in a violent crackdown on the narcotics trade. President Rodrigo Duterte, who unleashed the drug war after winning election in 2016, once promised to defend the policemen in the case, saying he was he was ready to go to jail for them. Police say more than 6,100 drug dealers have been killed by officers acting in self-defence during anti-drug operations since the president took office. Right activists say systematic cover-ups and executions of thousands of drug users and pushers alike have not been prosecuted. Police have denied wrongdoing. The judge who acquitted the 19 policemen of homicide on Wednesday was quoted by the Philippine Star newspaper as saying that prosecutors were not able to prove their guilt. Cristina Palabay of human rights group Karapatan criticised the outcome: "Domestic accountability mechanisms have been inadequate and unresponsive to the dire needs of victims for justice and accountability." In a rare admission by the state that abuses may have taken place, the Philippines" justice ministry said on Sunday it had found 154 police officers could be criminally liable over their conduct in the drug war following an initial investigation.Michelle Bachelet, the U.N. human rights chief, said in a statement: "Despite these steps, I remain disturbed by reports of continuing and severe human rights violations and abuses across the country, including killings by members of the security forces and law enforcement in counter-narcotics and counter-insurgency operations – often in circumstances that indicate basic human rights standards have been ignored." The International Criminal Court has approved a formal investigation into thousands of state killings of alleged drug dealers during Duterte"s tenure. The popular Duterte, 76, has dared the ICC to put him on trial and publicly said he would happily "rot in jail" for killing people intent on destroying his country.