Supporters of Philippine President Ferdinand Marcos and his predecessor Rodrigo Duterte gathered for rival rallies on Sunday, as a battle over the country's constitution highlighted a deepening rift between the powerful clans.

In the nearly two years since Marcos was 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.

As thousands massed at a seaside park in the capital Manila hours ahead of an early evening rally headlined by the president, his predecessor's youngest son Sebastian Duterte was openly calling for Marcos to step down.

"If you don't show love and aspiration for the country, better resign," city mayor Duterte, speaking in the Cebuano language in the family's southern bailiwick of Davao, told supporters, blaming Marcos for the drug and communist insurgency problems in the region.

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 both Dutertes have publicly opposed.

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

That would potentially put him on a collision course with Sara Duterte.

"Our constitution is fine... why do we give ourselves a headache when we know all along that this is in order to, you know, you want to stay in power," ex-president Duterte said in a recent interview.

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

Marcos will launch his "Bagong Pilipinas" (New Philippines) campaign at the Manila rally "to usher the nation towards change and development".

The Duterte patriarch, meanwhile, is expected to attend a separate rally in Davao city that has been organised by opponents of constitutional change.

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

Both events are expected to draw tens of thousands of supporters. 

"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 even though they admitted 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.

Remelyn Zuniga, 39, said: "We're here to listen to any help he can offer us. I'm a single mother and it would be good to get cash assistance."

Police said around 4,000 people arrived early for the Manila rally, listening to a free concert at the site.

The 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.

It 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. 

House committees also approved resolutions calling on the Marcos administration to cooperate with the International Criminal Court's investigation into Duterte's drug war.

Marcos has said he will not "lift a finger" to help the ICC probe, which he described as "a threat to our sovereignty". (Cecil MORELLA)

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© Agence France-Presse