MANILA -- Philippine President Rodrigo Duterte said on Saturday he was retiring from politics, a surprise move that fuelled speculation he was clearing the way for his daughter to run to succeed him.

"Today, I announce my retirement from politics," Duterte said, accompanying loyalist Senator Christopher "Bong" Go from their ruling PDP-Laban party as he registered to run for vice president.

Duterte had been expected to run for vice president. He is not eligible to run again for the top job as the constitution sets a single-six-year term limit for the president.

Political observers had long suspected Duterte could spring a surprise, such as a presidential run by his daughter, Sara Duterte-Carpio, next year.

Duterte-Carpio, who replaced her father as mayor of Davao, said last month she was not running for higher office next year because she and her father had agreed only one of them would run for national office in 2022.

The older Duterte's decision not to join the race next year would clear her way.

"This allows Sara Duterte to run," said Antonio La Vina, professor of law and politics at the Ateneo de Manila University. But La Vina said he could not rule out the possibility the firebrand leader could have a change of heart and be Go's substitute.

Candidates have until Friday to register, but withdrawals and substitutions are allowed until Nov. 15, leaving scope for last-minute changes of heart, like the 11th-hour entry of Duterte for the 2016 election, which he won by a huge margin.