MANILA - Presidential candidate and Davao City Mayor Rodrigo Duterte on Friday fired back at critics who brushed off his promise to stamp out criminality and drug trade in the country within three to six months.

"Umalis lang muna sila diyan and I will take over and I'll show them how to do it," Duterte said at a campaign sortie in Bukidnon province.

But prompted to expound on how he will deliver his promise, the PDP-Laban standard bearer merely said: "Secret, secret."

Duterte, who is running on a platform of transforming the country into a crime-free showcase, has been quoted as saying that he will order the police and the military to kill outlaws.

He added that he will claim responsibility for the order if complaints are filed against law enforcers.

READ: Duterte warns outlaws: 'I can eat your heart in front of you'

Philippine National Police (PNP) chief Director General Ricardo Marquez on Friday dismissed Duterte's promise as mere words.

Marquez also asserted that law enforcers should never allow themselves to be "executioners" and directly disobey their mandate to "serve and protect."

President Benigno Aquino III, meanwhile, alluded to Duterte's offer to resign from the presidency if he fails to curb crime and corruption.

While Aquino did not directly name Duterte, the former said being president is a commitment, and that the candidate should make up his mind if he wants the job.

READ: PNoy tells Duterte: Gusto mo ba o ayaw?

Reacting to this, Duterte quipped that his statement on resigning was just a joke.

"Naniwala pala siya (Aquino)? He's the president, nagbibiro lang ako," the 71-year-old mayor said.

Duterte also criticized Aquino's anointed successor, Mar Roxas, saying that the latter will fail to wipe out criminals as president.

"Kung ako si Roxas, naging presidente, 50 years, walang ma-achieve," Duterte said.

The mayor earlier attacked Roxas for being ineffective during his time as secretary of the Department of Transportation and Communications (DOTC) and chief of the Department of Interior and Local Government (DILG).

READ: Duterte calls Roxas 'pretentious, a weak leader'

The seven-term mayor, meanwhile, scoffed at a recent PNP report ranking Davao as the fourth city with the highest number of index crimes from 2010 to 2015.

SLIDESHOW: Top 15 cities with highest index crimes

Duterte pointed out that the DILG, which supervises the PNP, earlier awarded the city a certificate of good governance, a supposed proof of his turf's peaceful state. - Report from Angelo Andrade, ABS-CBN News Northern Mindanao