Armed clashes erupted Thursday in Beirut during a protest organized by the Lebanese militant group Hezbollah and its allies against the lead judge probing last year’s blast in the city’s port. At least six people were killed and dozens were wounded in some of the most serious fighting in years, authorities said.

The hours-long exchange of fire along a former front line from the 1975-90 civil war involved snipers, pistols, Kalashnikovs and rocket-propelled grenades, and were reminiscent of that conflict. The clashes were the worst since 2008, when the Shiite Hezbollah briefly overran parts of Beirut.

It was not immediately clear how Thursday’s violence erupted but tensions were running high after the Iran-backed Hezbollah and its Shiite allies from the Amal Movement demanded the removal of the judge leading the investigation into last year's massive port explosion. The two parties called for a protest near the Justice Palace, along the former front line between Muslim Shiite and Christian areas.

Read: Huge fire breaks out at Beirut port a month after explosion

In a statement Thursday, the two groups said their protesters came under fire from snipers deployed over rooftops in the Tayouneh area.

Gunfire echoed in the capital for several hours and ambulances, sirens wailing, rushed to pick up casualties. Snipers shot from buildings. Bullets penetrated apartment windows in the area. Four projectiles fell near a private French school, Freres of Furn el Chebbak, causing panic, a security official said, speaking on condition of anonymity because he was not authorized to speak to the press.

The students huddled in the central corridors with the windows open to avoid major impact, in scenes reminiscent of the 1975-90 civil war. Smoke covered the neighborhood where intense gunfire was relentless. A car caught fire, while a blaze was reported in a lower floor where residents were stuck and called for help.

Haneen Chemaly, a resident of Furn el-Chebbak and mother of a 6-month old girl, said she first moved to the corridor before running to the shelter because the sound of gunfire was terrifying from her 10th-floor apartment.

“I did it for my child,” she said. “I don’t know what is happening. I can just hear the sound of gunfire.”

The violence unfolded while U.S. Undersecretary of State for Political Affairs Victoria Nuland was in town, meeting with Lebanese officials. Her schedule was slightly thrown off by the action on the streets.

The demands for Bitar’s removal and calls for protest upset many who considered it blatant intervention in the work of the judiciary.

The right wing Christian Lebanese Forces mobilized supporters Wednesday evening after Hezbollah and Amal called for the protest at the Justice Palace, located in a Christian area. Videos circulating on social media showed supporters of the Christian Lebanese Forces marching in the streets, carrying large crosses.

A journalist with The Associated Press saw a man open fire with a pistol during Thursday's protest, as well as gunmen shooting in the direction of protesters from the balcony of a building. Several men fell immediately from the gunfire and bled on the street. The army deployed heavily and sent patrols to the area to search for the gunmen, following the exchanges of gunfire between the Muslim and Christian sides of the capital.

A staffer at the emergency room at al-Sahel hospital said they received three bodies and 15 people who were injured. One of the dead, a woman, had received a bullet to her head. Two of the 15 injured were in critical condition.

In a statement, Prime Minister Najib Mikati appealed for calm and urged people “not to be dragged into civil strife.”

The probe centers on hundreds of tons of ammonium nitrates that had been improperly stored at a port warehouse that detonated on August 4, 2020, killing at least 215 people, injuring thousands and destroying parts of nearby neighborhoods. It was one of the largest non-nuclear explosions in history and has further devastated the country already roiled by political divisions and unprecedented economic and financial meltdown.

Read: Huge fire extinguished at oil facility in southern Lebanon

Bitar is the second judge to lead the complicated investigation - his predecessor was removed following legal challenges. Now Bitar has come up against formidable opposition from the powerful Hezbollah group and its allies who accuse him of singling out politicians for questioning, most of them allied with Hezbollah.

None of Hezbollah’s officials have so far been charged in the 14-month-old investigation.

Sporadic shooting continued even after army troops deployed to the area Thursday. Residents and civilians in the area were ducking to avoid the shooting. Someone screamed: “Some martyrs on the ground!” People pulled one man who was apparently shot and down, away from the line of fire. Others pulled another body away.

In some videos circulating online, some men were chatting: “Shiite Shiite” on the streets, as residents were running from the gunfire.

The tensions over the port blast add to Lebanon's enormous multiple troubles, including an unprecedented economic and financial meltdown, an energy crisis leading to extended electricity blackouts, hyperinflation and soaring poverty.

Chemaly said said there was no electricity for her to follow on TV what was going on. So she knew nothing of the situation on the ground and opted for safety. After spending some time in the shelter, she moved to the first floor to stay with her neighbors away from the fire.

“I know there was so much mobilization from the night before, all predicting that a war would erupt,” Chemaly, who heads a local NGOs that provides social services. Civil war erupting “is the last card they have to use. They have (driven) us into bankruptcy, devastation and now they are scaring us with the specter of civil war.”

The armed clash could derail the country’s month-old government of Prime Minister Najib Mikati even before it begins tackling Lebanon’s economic meltdown.

A Cabinet meeting was canceled Wednesday after Hezbollah demanded urgent government action against the judge. One Hezbollah-allied minister said he and other Cabinet members would stage a walkout if Bitar isn’t removed.