Gunshots have been fired in the Philippines senate as a senator who is wanted by the international criminal court (ICC) remained holed up in the building to evade arrest.

Ronald dela Rosa, a Philippine senator accused of crimes against humanity for his role in overseeing the former president Rodrigo Duterte’s “war on drugs”, has spent two nights in the country’s senate in a standoff with the authorities.

On Wednesday he called for his supporters to gather in front of the senate, and said his arrest was imminent.

Military personnel were later seen entering the senate building, some carrying assault rifles. Local media showed scenes of chaos and the sound of gunfire rang out.

It is unclear who fired the shots. The country’s president, Ferdinand Marcos Jr, said in a statement late on Wednesday that no government personnel had been involved in the incident and he did not know who was responsible. He promised an investigation, questioning if the event was an attempt to “destabilise the government or trigger chaos”.

The interior secretary, Jonvic Remulla, said security footage would be reviewed. Referring to Dela Rosa by his nickname, he told media outside the building: “I will not arrest Senator Bato. I am here to secure everyone.”

“We do not know who is behind this,” Remulla said of the gunfire. “But we will find them.”

Dela Rosa, 64, who was the chief enforcer of Duterte’s merciless anti-drugs crackdown, dramatically evaded arrest on Monday when he outran government agents who chased him through the senate’s hallways and staircases. He managed to reach the chamber, where the senate president, Alan Peter Cayetano, an ally of Duterte, offered him protective custody.

Duterte was arrested last year in Manila and sent to the ICC, where he faces charges of crimes against humanity over the crackdowns, in which thousands were killed.

The senate secretary, Mark Llandro Mendoza, told reporters there had not been any casualties reported.

Dela Rosa has been in the senate since Monday’s pursuit, which was captured on the building’s CCTV. Appealing for the public to gather to protect him from arrest, he said earlier on Wednesday: “Let us not allow another Filipino to be taken to The Hague.”

He had called on the military to peacefully oppose moves to send him to The Hague, and sang part of the Philippine military academy hymn in front of a media scrum.

“I am not appealing for violent support. I am appealing for peaceful support,” Dela Rosa said, as he urged his “fellow men in uniform” to “express their sentiment” that Ferdinand Marcos Jr’s government “should not hand me over to foreigners”.

As the chaotic scenes unfolded on Wednesday night, Cayetano posted on Facebook from inside the building: “We don’t know what’s happening, everyone is locked in their rooms now, we cannot go out. We cannot secure our other staff,” he said.

“This is the senate of the Philippines,” he added, appealing to law enforcement agencies for information. “What is happening? Why are we under attack here?”

He said he and others were “not going to leave” Dela Rosa and legal remedies to avoid his transfer to The Hague had not yet been exhausted.

Dela Rosa has filed an emergency petition to the supreme court asking it to block any attempt to send him to the ICC. The court has given all parties to the petition 72 hours to respond.

“We heard that allegedly there are NBI [National Bureau of Investigation] members who want to enter. They are seen drilling door, walls,” Cayetano said.

However, Melvin Matibag, the NBI’s director, told local media he had not deployed any personnel to the senate, and that the gunfire should be investigated and those responsible arrested.

More than 10 military personnel in camouflage fatigues were present, some carrying assault rifles, according to Reuters. The chief of the military’s public affairs office, Xerxes Trinidad, told Reuters the senate had requested help to “assist them in securing the facility”.

The events follow several days of political drama in the Philippines, including the impeachment of the vice-president, Sara Duterte, Rodrigo Duterte’s daughter, on Monday. She is embroiled in a fierce battle with Marcos, which intensified last year when her father was arrested.

Dela Rosa is accused by the ICC of the crime against humanity of murder along with Duterte and other co-perpetrators. He did not respond to a request for comment on Wednesday, but has denied wrongdoing.

An arrest warrant accuses him of “authorising, condoning and promoting” the killings, providing weapons, promising impunity and rewarding perpetrators, according to an ICC arrest warrant that was unsealed on Monday.

Reports that Dela Rosa could face arrest first surfaced in November and he had been missing from the senate for months. However, he made a surprise appearance on Monday, when he turned up to back a successful attempt to install Cayetano, a staunch ally of the Duterte family, as senate president.

The ICC, not to be confused with the UN’s international court of justice, deals with accusations of war crimes, genocide and crimes against humanity and is made up of 125 member states.

Reuters contributed to this report