Philippine President Benigno Aquino warned Thursday his country was on the brink of another dictatorship, in a thinly veiled reference to controversial politicians who lead polls to win the top two posts in next month's elections.

Tough-talking Rodrigo Duterte has emerged as a clear favorite in polls for the presidential election, with voters apparently embracing his vows to kill tens of thousands of criminals outside the judicial system.

Meanwhile, the son and namesake of dictator Ferdinand Marcos is leading the vice-presidential race with a campaign strategy of highlighting the alleged benefits of his father's rule.

"This is the tragedy poised to befall our nation: That we were oppressed before and yet will allow ourselves to be oppressed again and suffer the same problems that we endured under the dictatorship," Aquino said in a speech in Manila.

"Our successes could vanish, especially if my successor would stray from our straight and narrow path. The question is, will you allow these bullies who make enemies each time they open their mouth to become president?"

Aquino did not mention Duterte by name, but it was a thinly veiled reference to him, as the presidential favorite's opponents often call him foul-mouthed and a bully.

Aquino did, however, specifically mention the younger Marcos, criticizing him for refusing to apologize for his father's crimes.

The elder Marcos' two-decade rule ended in 1986 as millions took to the streets in a famous "People Power" revolution led by Aquino's mother.

Marcos was accused of plundering billions of dollars from state coffers and overseeing widespread human rights abuses.

But the Marcos family, exiled to the US after the revolution, has staged a remarkable political comeback that would be cemented if the former dictator's son were elected vice president.

In the Philippines, presidents can only serve a single term of six years. Aquino's preferred successor, Mar Roxas, is trailing in the polls.