Senator Ronald "Bato" dela Rosa, the former national police chief and top enforcer of ex-president Rodrigo Duterte's bloody "war on drugs", has been under Senate protection and is wanted for crimes against humanity, the same charges Duterte is accused of.
"Several sources confirmed that the senator, Senator Bato, is no longer in the Senate premises, but we are still getting confirmation," presidential spokeswoman Claire Castro said.
Gunshots were heard late on Wednesday inside the Senate, hours after dela Rosa, 64, appealed on social media for supporters to mobilise, saying law enforcement agents were coming to arrest him.
The incident sparked chaos, with a heavy presence of police and armed guards at the Senate, protests outside and more than a dozen shots fired after marines were called in to bolster security.
President Ferdinand Marcos Jr met security chiefs on Thursday and police said one person had been detained, and bullet casings and assault rifle magazines recovered.
It was unclear who fired the shots, or if dela Rosa was still in the Senate on Thursday, with speculation rife that he had slipped away overnight.
The Hague-based court unsealed a warrant on Monday for his arrest, while he has filed an emergency appeal, arguing the ICC has no jurisdiction in the Philippines after its 2019 withdrawal from the international institution.
Earlier on Thursday, while entering the heavily guarded Senate, dela Rosa's lawyer Jimmy Bondoc said he spoke to him during the night and believed he was inside.
"I asked him if you have plans to leave, he said none," Bondoc told reporters.
The tough-talking dela Rosa enjoyed celebrity status as Duterte's top lieutenant, overseeing a fierce crackdown during which thousands of alleged drug dealers were slain, with rights groups accusing police of systematic murders and cover-ups.
Police reject that and say the more than 6000 killed in Project Double Barrel were all armed and had resisted arrest.
Marcos has sought to distance his government from the Senate drama and insists no order was given to apprehend dela Rosa, who on Tuesday appealed to the president not to arrest him.
In a radio interview, dela Rosa - who denies involvement in illegal killings - said he would "exhaust all available remedies" to block his ICC transfer and having learned about conditions Duterte was being held under, he was no longer willing to fight his case in The Hague.
The Senate stand-off is a major challenge to Marcos, who relied on support of the influential Duterte family to win a 2022 election before a falling out that led to him handing over predecessor Rodrigo Duterte to the ICC.
Political tension has mounted in recent days over dela Rosa and Monday's impeachment of the former president's daughter, Vice-President Sara Duterte, Marcos' former ally and running mate.
