A burst of gunfire rang out Wednesday night in the Philippine Senate, where authorities have tried to arrest a senator who is wanted by the International Criminal Court on a charge of crime against humanity, an Associated Press journalist and other witnesses said.

It wasn't immediately clear what set off the gunfire or if there were injuries in the Senate, where Sen. Ronald dela Rosa has stayed in protective custody by allied senators as Philippine authorities tried to arrest him and possibly turn him over later to the ICC.

Senate President Alan Cayetano briefly appeared before journalists in the Senate and confirmed that he has been told by the building's security that gunshots were fired, but he didn't provide other details and hastily left.

"The emotions are high here," Cayetano said. "This is the Senate of the Philippines and we are allegedly under attack."

On Monday, the ICC unsealed an arrest warrant for dela Rosa, a former national police chief who first enforced then President Rodrigo Duterte's anti-drug crackdowns, in which thousands of mostly petty suspects were killed.

Originally issued in November, the warrant charges dela Rosa with the crime against humanity of murder of "no less than 32 persons" between July 2016 and the end of April 2018, when he led the national police force under Duterte.

Dela Rosa, 64, has vowed to fight the ICC arrest order and said that he would seek all legal remedies. He also called on his followers on Wednesday night to gather in the Senate to prevent what he said was his impending arrest.

National Bureau of Investigation agents tried to arrest dela Rosa on Monday, but he managed to dash to the Senate's plenary hall and sought the help of fellow senators. Cayetano said then that he would cite the government agents involved for contempt.