Thousands of supporters of Philippine President Ferdinand Marcos and his predecessor Rodrigo Duterte gathered for rival rallies on Sunday, putting on a public display of a deepening rift between the powerful clans.

In the nearly two years since Marcos swept to victory with the help of his vice presidential running mate Sara Duterte -- the former president's daughter -- their so-called "Unity Team" has fractured.

After Marcos launched his "Bagong Pilipinas" (New Philippines) campaign to develop the country and make the government more accountable, his predecessor accused him of being a "drug addict" and of trying to change the constitution to prolong his term in office.

Sara Duterte briefly appeared at the Marcos rally in Manila, saying the education department that she also heads was "one with all the other government agencies in pushing for a New Philippines" -- before flying to her family's stronghold of Davao city for the rival event.

Relations between the families have deteriorated as they seek to shore up their respective support bases and secure key positions ahead of next year's mid-term elections and the 2028 presidential race, which Sara Duterte is widely expected to contest.

In the latest rift, Marcos has backed a campaign for the 1987 constitution, introduced after his dictator father and namesake was ousted from power, to be changed to allow in more foreign investment -- something the Dutertes have publicly opposed.

Critics warn the effort could pave the way for Marcos to seek another six-year term, which is currently prohibited.

That would potentially put him on a collision course with Sara Duterte for the top job.

At the Davao rally organised by opponents of constitutional change, the Duterte patriarch warned Marcos that "you may suffer your father's fate" and called on the police and military to "protect the constitution".

Earlier, Duterte's youngest son Sebastian openly dared Marcos to step down.

"If you don't show love and aspiration for the country, better resign," Sebastian told supporters, blaming Marcos for the drug and communist insurgency problems in the region.

Marcos has said the constitution's political provisions, including term limits for public officials, should be tackled later.

An ever-shifting coalition of powerful families has long ruled the Philippines, holding onto power for generations.  

Left-wing political party Akbayan branded the rallies as a "dynastic war among ruling elites" that did not serve the interests of regular Filipinos.

Police estimated 400,000 people were at the Manila rally, many of them government workers, while more than 40,000 were at the Davao event. 

"This would be the most public and deliberate way of letting the people know that the Unity Team is no more," University of the Philippines political science professor Jean Franco told AFP.

Unravelling alliance

In Manila, some members of the crowd told AFP they believed in Marcos, although they said they knew next to nothing about the constitution controversy.

"If he runs again why not, for as long as he is doing a good job for the country," Manila resident Dennis Ardea, 53, told AFP.

"The Dutertes belong to the past. We should go with another (leader)," Ardea added.

The unlikely alliance between the two families began to unravel soon after the 2022 election, with Marcos giving Duterte the problem-plagued education portfolio instead of her preferred post of defence secretary.

The deterioration has gathered pace since then, with the powerful House of Representatives Speaker Martin Romualdez, who is Marcos's cousin and also widely expected to seek the presidency in 2028, demoting influential congresswoman Gloria Macapagal Arroyo, a former president and close ally of Sara Duterte.

Romualdez also spearheaded efforts to strip the vice presidency and the education department of millions of dollars in special confidential funds.

And the House, where Marcos has a majority of support, has pushed for a pro-Duterte radio and television broadcaster to be stripped of its licence after one of its hosts made accusations against Romualdez over his travel budget.