Duterte had requested to be released citing health reasons. But the request was denied on Friday.
The 80-year-old is accused of crimes against humanity. The trial concerns ordered killings in connection with his so-called "war on drugs" from 2011, when Duterte was mayor of Davao City, until 2019, when he had been president for three years.
Mostly poor men were targeted, even without evidence of drug connections.
The Philippine National Police has stated that more than 6,000 have been killed, while human rights groups estimate that it is more likely around 30,000.
