Anti-establishment Philippine politician Rodrigo Duterte's rollicking ride to presidential favouritism has triggered warnings of a coup should he win next week's election, with opponents warning he is a dictator in the making.

The Philippines has endured a tumultuous democracy since millions of people took to the streets to overthrow dictator Ferdinand Marcos in 1986, with a succession of leaders having to quell military unrest and one president ousted in another uprising.

Duterte is enjoying a double-digit lead over his rivals ahead of Monday's presidential election, but he has created enemies with vows to embrace communist rebels and threats to abolish Congress or create a revolutionary government that could rewrite the constitution.

"The moment he tries to declare a revolutionary government, that is also going to be the day he will be removed from office," Senator Antonio Trillanes, a former navy officer famous for leading failed military uprisings in 2003 and 2007, told AFP.

"This guy has no respect for democratic institutions."

Trillanes said some in the military were "strongly averse" to Duterte's long-standing ties with communists, and that a coup was "very likely".

Communists in the Philippines are waging one of Asia's longest-running insurgencies, with tens of thousands of killed since the rebellion began in 1969. Dozens of people still die each year, with the communists retaining support among the poor.

Duterte has ruled the major southern city of Davao, which was one of the communist hotspots, as mayor for most of the past two decades, ending violence there by forging close ties with the rebels.

Duterte has vowed to offer communist leaders posts in his government.

He has also raised deep fears about the rule of law under his presidency, promising to kill tens of thousands of criminals and pardon himself for mass murder.

President Benigno Aquino, who is limited by the constitution to a single term of six years, has spoken out repeatedly in recent weeks about his concerns that Duterte could turn into a dictator.

"Now that we are free, people who act like dictators are the ones in the lead," Aquino said on Wednesday, as he warned the gains of democracy were in jeopardy.

Ashley Acedillo, another coup plotter-turned-lawmaker, also told AFP that a "military intervention" was likely under a Duterte presidency.

"The armed forces will stand true to its constitutional duty to protect the people and the state," he said.

The ex-coup plotter's warnings are not bluster, according to Manila-based security analyst Rommel Banlaoi.

"We will face a Duterte government that is very unstable," Banlaoi told AFP.