The fiery populist politician has cultivated an image as a no-nonsense leader, vowing to kill 100,000 criminals.
Duterte, a fiery populist politician who was elected by a landslide earlier this year largely on a vow to kill 100,000 criminals, has cultivated an image as a no-nonsense leader.
He has made reviving the death penalty in the mainly Catholic nation his top legislative priority as part of his war on crime, and has likened himself to Nazi leader Adolf Hitler as he said he was "happy to slaughter" three million drug users.
Speaking during a visit to the northern Philippines on Thursday, the 71-year-old lamented how the constitution tied the president's hands in dealing with security crises including war.
"If you have martial law, only one person should be in control," Duterte said.
"If there's invasion or war and I declare martial law, I cannot proceed on and on to deal with the trouble as I still have to go to Congress, go to the Supreme Court," he added.
"That's why that needs to be replaced."
The Philippines adopted a new constitution in 1987 to curtail presidential powers after millions of Filipinos took to the streets the year earlier in a famous "People Power" revolution, to oust dictator Ferdinand Marcos and end his 20-year rule.
Under the former leader, who imposed martial rule from 1972-1981 to fight crime and a communist insurgency, thousands were killed and tortured to suppress dissent, previous Philippine governments have said.
Today the president can impose martial rule for up to 60 days to stop invasion or rebellion, but parliament can revoke it within 48 hours, while the Supreme Court can also review its legality.
Bonifacio Ilagan, imprisoned and tortured under Marcos' martial law reign, said Duterte could be floating a "trial balloon" to gauge public opinion before taking actual steps to amend the constitution.
"I honestly believe that the people will resist," said Ilagan.
Asked to explain Duterte's intentions, spokesman Martin Andanar told AFP on Friday: "I will ask the president."
Duterte has spent his first six months in office waging a brutal campaign against drugs that has left more than 5,300 people dead and raised concerns over alleged extrajudicial killings.
The president has previously declared he does not need martial law, but has also threatened to impose it during a row in August with the chief justice of the Supreme Court who had criticised his drug war.
Manila Auxiliary Bishop Broderick Pabillo, who ministers to the Tondo slum district where many drug suspects have been killed by police, told AFP the country was now under virtual martial rule due to the crackdown.
"It is not necessary that you have a declaration of martial law to have martial law," the bishop said.
Another prominent critic, Senator Francis Pangilinan, said Duterte's shifting position on martial rule was not reassuring.
"He said a few days ago that martial law was stupid and didn't work, and yet now he says something else. His lack of clarity is a serious cause for concern."
Duterte's allies who control parliament have backed his proposal for it to convene as a "constituent assembly" before he leaves office in 2022 to change the centralised government to a federal system.
Ilagan said the constituent assembly would also be able to amend the president's martial law powers.