Senator Ronald dela Rosa, the former national police chief and chief enforcer of ex-president Rodrigo Duterte's bloody war on drugs, is wanted by the International Criminal Court (ICC) for crimes against humanity.
Duterte faces the same charges and is set to become the first former Asian head of state to go on trial at the international court.
Gunshots were heard late on Wednesday inside the Senate building and people inside scrambled for cover, hours after dela Rosa, better known as "Bato" or "Rock", appealed on social media for supporters to mobilise, saying law enforcement agents were coming to take him from the legislature, where he had taken refuge.
The incident sparked chaos and confusion, with a heavy presence of police and armed guards at the Senate, protests outside and more than a dozen shots fired just moments after marines were called in to bolster security.
President Ferdinand Marcos Jr was meeting government and security chiefs on Thursday, while police spokesperson Randulf Tuano said one person had been detained and investigations were under way, with bullet casings, assault rifle magazines and other items recovered.
"The person has provided names, but these still need confirmation," Tuano told DZBB radio.
It was unclear who fired the shots, or if dela Rosa was still in the Senate on Thursday.
Entering the heavily guarded building, his lawyer Jimmy Bondoc said he spoke to dela Rosa during the night and believed he was inside.
"As his lawyer, I asked him if you have plans to leave, he said none," Bondoc told reporters.
Marcos and government agencies have insisted no order was issued to arrest dela Rosa, with no announcements on Thursday on the identity of individuals who officials said tried to force their way into the building.
In an interview with DZBB aired early on Thursday, dela Rosa said he will "exhaust all available remedies" to block his transfer to the ICC and having learned about conditions Duterte was being held under, he was no longer willing to fight his case in The Hague.
Dela Rosa has denied involvement in illegal killings.
"Yes, things changed. It turns out it is not that easy to visit him," he said of Duterte. "If we were co-detainees there's no assurance we would be placed in the same cell or even in the same facility."
Marcos vowed late on Wednesday to get to the bottom of the incident, as political tensions mounted over dela Rosa and Monday's impeachment of the former president's daughter Vice President Sara Duterte.
Sara Duterte, who is in The Hague visiting her father, was the running mate of Marcos, but their alliance broke down and she is now fighting for her political survival, facing an impeachment trial in the Senate that could derail her run for the next presidency in 2028.
