President Rodrigo Duterte yesterday claimed that narcotics syndicates and mining companies are financing efforts to topple his eight-month-old government.

"I know (the mining companies) are active in destabilisation… I know they are funding the opposite side," Mr Duterte, 71, said at a three-hour-long news conference yesterday.

"Mining lobby money is coming in. They've been contributing a lot. They are still financing her, drug money and mining," he added, referring to Senator Leila de Lima, the most vocal critic of his brutal war on drugs.

Ms De Lima, 57, is being detained inside the police's national headquarters in Manila on charges she accepted bribes from drug lords serving time at the national penitentiary.

Her supporters claim she has been targeted for political persecution as she has led investigations into Mr Duterte's human rights records.

More than 6,500 people suspected of being drug dealers or users have been killed by the police and vigilantes since Mr Duterte took office in June last year.

The mining industry has been lobbying in Congress to boot out Mr Duterte's pick to head the Department of Environment and Natural Resources, Ms Regina Lopez.

Ms Lopez, 63, a fiery environmental crusader, last month ordered the closure of 23 of the country's 41 mines and suspended five others to protect watersheds after a months-long review by the environment agency.

Mr Duterte has largely backed her efforts.

For nearly an hour yesterday, he showed photos, tables, graphs and studies showing damages caused to the environment by open-pit mining.

"How can you argue with Gina when she's confronting you with a visual case?" he said.