A car bomb killed four people in south Beirut Thursday, the fourth attack to hit the Hezbollah bastion since the Shiite group announced its intervention in Syria last year, the health minister said.
The bombing came just weeks after a twin suicide bombing killed 25 people at the Iranian embassy in the same area and marked a new breach of the tight security in Hezbollah's stronghold.
Health Minister Ali Hassan Khalil said four people had been killed and 77 wounded. He said the remains of a fifth person had also been found.
Interior Minister Marwan Charbel said it may have been a suicide bombing.
Hezbollah's second-in-command, Sheikh Naim Qassem, called for unity in order to avoid the "destruction" of Lebanon.
The militant group's public confirmation last April that its fighters had intervened in the civil war alongside President Bashar al-Assad's forces outraged Lebanese Sunnis, most of whom sympathise with the Syrian rebels, and has made it a target for Sunni hardliners.
An AFP photographer in the densely populated Haret Hreik area saw flames and smoke rising from burning vehicles and at least three damaged buildings.
Hezbollah's Al-Manar television aired footage of panicked bystanders scrambling to douse burning vehicles on the busy Al-Arid street, beneath a building whose facade had been burned out.
"The terrorist explosion targeted a densely populated residential area, just 150 to 200 metres (yards) away from Hezbollah's political bureau," Al-Manar reported, but said the building was not thought to have been the target.
The Hezbollah deputy leader did not accuse any party of being behind the attack, but did make a call to face "takfiri (radical Sunni extremist) terrorists," echoing accusations by Hezbollah over previous attacks.
"Lebanon is on the path to destruction if there is no political understanding, and we cannot save (the country) without national unity," Qassem told Al-Manar.
Haret Hreik is symbolic for Hezbollah, which once based many of its leadership institutions in the area.
Much of the neighbourhood was reduced to rubble during the massive Israeli air bombing that accompanied its 2006 war with Hezbollah, but it has since been rebuilt.
Interior minister Charbel told private Lebanese channel MTV: "We are leaning towards the hypothesis that a suicide bomber" caused the blast.
Signup to our newsletter and follow us on Facebook and Twitter!
The Lebanese army said 20 kilogrammes (44 pounds) of explosives had been planted inside a four-by-four vehicle, and that "the method of explosion is being investigated."
In Lebanon's main northern city Tripoli, scene of frequent Syria-related violence, a firefight broke out pitting Sunnis against Alawites after the Beirut blast, killing one woman and wounding four other people, a security source said.
President Michel Sleiman said the car bomb had been planted "by the same hand that plants terrorism, killing and destruction everywhere in Lebanon."
He called for dialogue among the country's divided leaders and urged security services to bring the perpetrators to justice.
Caretaker prime minister Najib Mikati said "the hand of terrorism does not differentiate between us... Rather, it is planning a despicable conspiracy to drown Lebanese in sectarian strife."
The US State Department condemned the "terrorist bombing."
"We urge all parties to refrain from retaliatory acts that would further escalate tensions and threaten Lebanon’s stability and the lives and livelihoods of the Lebanese people," said deputy spokeswoman Marie Harf.
UN chief Ban Ki-moon said the bombing "further reflects a deeply worrying escalation in the violence witnessed in Lebanon in recent months."
Thursday's bombing was the fourth attack in south Beirut since Hezbollah announced it was fighting in Syria.
As well as the Iranian embassy attack, the southern suburbs also suffered an August 15 bombing that killed 27 people and a blast earlier the same month that wounded some 50 people.
It comes less than a week after a car bomb in the heart of Beirut killed eight people, including prominent Sunni politician and former finance minister, Mohammad Chatah.
Former premier Saad Hariri, to whom Chatah was a key adviser, condemned Thursday's blast as a "diabolical act."
Analyst Lina Khatib, head of the Carnegie Middle East Centre, said the latest attack illustrated the graveness of the Syrian conflict's impact on Lebanon.
"Lebanon is entering a dangerous phase of attacks and counter-attacks that are increasing in frequency," she said.