BEIRUT -- A powerful car bomb tore through a bustling south Beirut neighborhood that is a stronghold of Hezbollah on Thursday, killing at least 18 and trapping dozens of others in an inferno of burning cars and buildings in the bloodiest attack yet on Lebanese civilians linked to Syria's civil war.
The blast is the second in just over a month to hit one of the Shiite militant group's bastions of support in years, and the deadliest in decades. It raises the specter of a sharply divided Lebanon being pulled further into the conflict next door, which is being fought on increasingly sectarian lines pitting Sunnis against Shiites.
Syria-based Sunni rebels and militant Islamist groups fighting to topple Syria's President Bashar Assad have threatened to target Hezbollah strongholds in Lebanon in retaliation for intervening on behalf of his regime in the conflict.
Thursday's explosion ripped through a crowded, overwhelmingly Shiite area tightly controlled by Hezbollah, turning streets lined with vegetable markets, bakeries and shops into scenes of destruction and burning cars.
Dozens of ambulances rushed to the scene of the explosion and fire fighters used cranes and ladders in trying to evacuate dozens of residents from burning buildings. Some terrified residents fled to the rooftops of buildings and civil defense workers were still struggling to bring them down to safety several hours after the explosion.
The blast appeared to be an attempt to sow fear among the group's civilian supporters and did not target any known Hezbollah facility or personality.
Hezbollah's Al Manar TV and Red Cross official George Kattaneh said the death toll was at least 18 and said more than 280 were wounded.
The army, in a statement, said the explosion was caused by a car bomb. It called on residents to cooperate with security forces trying to evacuated people trapped in their homes.
Syria's conflict has spilled across the border into its neighbor on multiple occasions in the past two years. Fire from Syria has hit border villages, while clashes between Lebanese factions backing different sides have left scores dead.
But direct attacks against civilian targets were rare until Hezbollah stepped up its role in Syria. Since then, its support bases in southern Beirut have been targeted. Since May, rockets have been fired at suburbs controlled by the group on two occasions, wounding four people. On July 9, a car bomb exploded in the nearby Beir al-Abed district, wounding more than 50 people.
However Thursday's explosion was much deadlier than those, the bloodiest single attack in south Beirut since a 1985 truck bomb assassination attempt targeting top Shiite cleric Mohammed Hussein Fadlallah in Beir al-Abed left 80 people dead.
It came despite rigorous security measure taken in the past few weeks by Hezbollah around its strongholds, setting up checkpoints, searching cars and sometimes using sniffer dogs to search for bombs. It also came a day before Hezbollah leader's was scheduled to give a major speech marking the end of the month-long 2006 war with Israel.
The explosion occurred on a commercial and residential main street in the Rweiss district, about 100 meters (yards) away from the Sayyed al-Shuhada complex where Hezbollah usually holds rallies.
Hezbollah leader Sheikh Hassan Nasrallah, who has lived in hiding since his group's 2006 month-long war with Israel, made a rare public appearance at the complex on Aug. 2, where he addressed hundreds of supporters. He was to speak again on Friday from a location in southern Lebanon, but his speeches by satellite are often transmitted to followers at the complex.
Panicked Hezbollah fighters fired in the air to clear the area and roughed up photographers, smashing and confiscating some of their cameras following the explosion.
Sunni-Shiite tensions have risen sharply in Lebanon, particularly since Hezbollah raised its profile by openly fighting alongside Assad's forces. Lebanese Sunnis support the rebels fighting to topple Assad, a member of a Shiite offshoot sect.
The group's fighters played a key role in a recent regime victory in the town of Qusair near the Lebanese border, and Syrian activists say they are now aiding a regime offensive in the besieged city of Homs.
A previously unheard-of group calling itself Aisha the Mother of Believers Brigades claimed responsibility for the attack in a video posted on YouTube, saying it is the second "message" they sent since last month's blast in the area. The authenticity of the claim could not be independently verified.
"Our second message was strong and astounding," said a masked man who read the statement, flanked by two other armed and masked men. He called on civilians to stay away from Hezbollah strongholds in the future, saying the militant group is "an agent for Iran and Israel."
Hezbollah lawmaker Ali Ammar called the blast a "terrorist" attack and called for restraint among the group's supporters. He suggested the group's political rivals in Lebanon were responsible for creating an atmosphere that encourages such attacks.
Politicians within Lebanon's Western-backed coalition have slammed the group for its involvement in Syria and called for its disarmament.
The U.S. Ambassador to Lebanon Maura Connelly strongly condemned the bombing. In comments posted on the embassy's Facebook page, Connelly called for all parties to exercise calm and restraint.
The British Foreign Office official in charge of Middle East policy, Alistair Burt, also condemned the attack.
"Terrorism and extremism have no place in Lebanon. I call for the Lebanese state to investigate this urgently and bring the perpetrators to justice," he said in a statement.
Outgoing Lebanese Prime Minister Najib Mikati declared Friday a day of mourning for the victims of the attack.