Philippine President Rodrigo Duterte told the army and police to shoot him if he becomes a dictator or stays beyond his term.

The firebrand leader made the solemn warning on Monday to eliminate speculation that he ordered loyalists in Congress to change the constitution.

His critics have raised concerns about the possible introduction of a federal system which would let him stay in power beyond 2022 when his single term ends.

'If I overstay and wanted to become a dictator, shoot me, I am not joking,' Duterte told soldiers during an army base visit.

President Rodrigo Duterte (pictured while giving a speech in Manila in 2016) told the army and police to shoot him if he becomes a dictator to eliminate speculation that he wants to change the constitution so he can stay beyond his single term

He added that security forces should not allow anybody to alter the constitution.

'It is your job to protect the constitution and to protect the people. Remember, it is your solemn duty,' he said.

Duterte has advocated federalism to tackle inequality, empower provinces and recognise the country's diverse makeup.

But a spokesman for the firebrand leader (pictured arriving for a meeting with Cambodia's Prime Minister in 2016) said the president does not want to stay longer than his term and, if anything, would prefer to retire earlier

The president's lower house allies voted last week to convene a constituent assembly to revise the charter by May this year.

This would scrap mid-term elections next year and extend the terms of all elected officials.

Constitutional reform has been a divisive issue in the country.

Critics have accused law-makers of trying to prolong their stay in office or find a way to keep hugely popular Duterte in power when his term ends.

Opponents warn that such a move could lead to a repeat of the oppressive rule of late dictator Ferdinand Marcos.

They say they are troubled by Duterte's admiration for Marcos and his similar authoritarian traits.

But Duterte's spokesman Harry Roque has repeatedly said the president does not want to stay longer than his term and, if anything, would prefer to retire earlier.