The critics said it would further heighten the "culture of impunity" in the country where law-enforcers have long been accused of killing people without fear of punishment.
But President Rodrigo Duterte's spokesman Ernesto Abella on Friday defended the reinstatement of Superintendent Marvin Marcos and 18 of his men, saying they were only following the rule of law.
"He (Marcos) was simply suspended. Basically that was the process: he was suspended, he served his suspension and he is back on active duty," Abella told reporters.
Marcos and his men are facing charges of homicide but were allowed out on bail. After serving a mandatory suspension period, they returned to police duties.
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Espinosa, mayor of the central town of Albuera, had previously surrendered to the police after Duterte accused him of involvement in illegal drugs.
Earlier this week, Duterte had called for the reinstatement of Marcos, reiterating he would not allow any policeman to be jailed for carrying out his orders, such as in his bloody war on drugs.
Fellow Senator Richard Gordon, said: "it is very, very wrong on the part of the president because he should not interfere in the justice system."
"Mr. President, these men are not your fellow uniformed servicemen," he said in a television interview.
Human Rights Watch criticised the reinstatement as well, saying: "the kid-gloves handling of the officers is emblematic of the wider impunity enjoyed by those responsible for the killing of more than 7,000 people in Duterte's murderous 'war on drugs'."
Police say at least 3,200 people were slain in their anti-drug operations but rights groups charge that thousands more have been killed by vigilante groups.