Philippine activists and the families of victims in a drug war that has killed thousands view with dread the possibility of President Rodrigo Duterte staying on as vice-president for six more years after his time in the top job ends next year.

The constitution bars a re-election bid by Duterte, 76, but he said on Tuesday he would run for vice president in 2022 in order to keep up his crusade on insurgency, criminal activities and drug dealers.

"For four years, we have been waiting for justice," said Randy Delos Santos, who believes that if Duterte succeeds it would make justice even more elusive for families such as his that have lost a member to the anti-drug campaign.