A powerful car bomb ripped through a busy shopping street in Hezbollah’s stronghold in the southern suburbs of Beirut on Thursday, killing at least 21 people and injuring hundreds in the deadliest attack to hit the Lebanese capital in more than eight years.

The explosion early Thursday evening tore the facades off apartment buildings and set afire parked cars in the Ruwais neighborhood, an area of staunch support for Hezbollah, the Lebanese Shiite movement that has been the subject of reprisal attacks since sending its militants in recent weeks to back President Bashar al-Assad’s forces in Syria. A group that asserted responsibility for the attack threatened “more and more” to come.

With Lebanon deeply divided over the Syrian civil war, Hezbollah has appeared to be trying to divert attention from its controversial decision to send fighters to Syria by focusing on its sworn enemy Israel, claiming on Wednesday an attack that wounded four Israeli soldiers last week. However, Syrian rebel groups have vowed to continue to strike against the movement until it withdraws its forces from Syria.

The Beirut explosion, which the Lebanese army said was caused by a car bomb, occurred just a few hundred yards from the site of a blast that injured more than 50 people just over a month ago.

Thursday’s bomb hit a packed shopping street, killing at least 21 people and injuring 336, the state news agency said Friday, citing caretaker Public Health Minister Ali Hasan Khalil.

It was the most lethal blast to hit the city since February 2005 assassination of Lebanon’s then prime minister, Rafiq al-Hariri, who was killed along with 21 others when a bomb hit his motorcade on Beirut’s seafront. If Thursday’s death toll climbs higher it will be the deadliest explosion since Lebanon’s 1975-1990 civil war.

The force of the blast cracked glass as far as 500 yards away. Roads around the area were blocked by Hezbollah plainclothes security personnel. A few hundred yards from the crash site, a man with a bloodstained white T-shirt manned a makeshift checkpoint. He said he had been helping to evacuate the wounded.

“There were more than you can count,” he said, declining to give his name.

Witnesses said dozens of residents were trapped in damaged buildings and had to be evacuated by ladder as the area was engulfed in flames. Ambulances and firetrucks screamed through the deserted streets as night fell.

“May Allah punish them,” said Kholoud Mahdi, a middle-aged Iraqi woman as she sat sobbing outside a shop near the blast site. “I was just here buying soap. I can’t hear from the explosion,” she said, clutching her left ear.

“We were expecting this, and we expect more,” said Mohammed Haider, 27, an Arabic literature student who lives nearby. “I’m sad for the victims who are my family, my neighbors, but this makes us more determined. Hezbollah needs to be in Syria to stop these attacks against its back. We are not afraid.”

A group calling itself the “external operations” arm of the Battalions of Aisha Um al-Mumineen asserted responsibility for the bombing in a video posted online.

“We send you, O pig called Nasrallah, you and your party, our second message strong and loud,” a masked man said in the video, referring to Hezbollah leader Hasan Nasrallah. The masked man, flanked by two others holding rifles, said Hezbollah had failed to “understand” the first message, seemingly a reference to last month’s bombing and a veiled threat against the group’s operations in Syria. “This is the second time that we decide the place and timing of the battle, and we will always be there inside your own homes, a disaster to you, and you’ll continue to see more of this.”

However, some Lebanese politicians pointed the finger at Israel, linking the bombing to Hezbollah’s claim Wednesday night that it had planted explosives that wounded four Israeli soldiers as they allegedly crossed into Lebanese territory in violation of a U.N. Security Council resolution.

Caretaker Prime Minister Najib Mikati declared that Friday would be a national day of mourning for the victims. “This blast indicates that the hand of evil is still tampering with this country and its security, safety and all assets,” he said in a statement.

Ahmed Ramadan contributed to this report.