A powerful explosion in the southern suburbs of the Lebanese capital Beirut - stronghold of the Shia armed group Hezbollah - has killed at least six people and left another 66 wounded.

The car bombing occurred during Thursday's rush hour in the city's Haret Hreik neighbourhood, but casualty figures were expected rise given the size and the timing.

Lebanon has been hit by a wave of bombings in recent months as the civil war in Syria increasingly spills over into its smaller neighbour.

Al Jazeera's Rula Amin, reporting from Beirut, said the explosion was a "major violation" of Hezbollah's security in the area as it was home to many buildings and officials belonging to the group.

"There are a large number of people trying to get to the explosion site but security forces are trying to keep people away," she said.

"They fear they might be another bomb, another attack."

Images broadcast on Hezbollah's Al-Manar TV showed smoke billowing at the site of the blast, and firefighters putting out several cars that were ablaze.

The footage showed at least one building that had its facade blown off, and several neighbouring buildings were also damaged.

"This is the heart of the Hezbollah stronghold and it's considered a safe zone but not anymore," our correspondent said.

"It seems like the bomb is not big but this area is very crowded and there are a lot of people on the streets at this time of the day.

"Security in that area is usually very tight ... so to be able to put that car bomb there is a major violation."

Speaking to Al Jazeera, Ibrahim Mousawwi, head of Hezbollah's media relations, said he did not want "to rush into any accusations" about who was responsible of the attack.

"We are living in a very volatile time in Lebanon," he said. "A political agreement is crucial."

'Big battle against terrorism'

Condemning the attack in an interview to Al Jazeera, Ali Hassan Khalil, the Lebanese health minister, said: "This is a big battle against terrorism, it targets everyone. It doesn't matter where they are from.

"The perpetrators are trying to incite violence among the Lebanese people."

Thursday's explosion came a week after a car bombing killed Mohamad Chatah, a former finance minister, and six others.

At least 71 people were wounded in the blast, which struck the residential Ain el-Merasa district of Beirut.

Saad Hariri, the former prime minister, and his March 14 coalition accused Hezbollah and Syria of involvement in the killing of Chatah, Hariri's 62-year-old political adviser.

Chatah's killing occurred less than a month before the long-delayed opening of a trial of five Hezbollah suspects indicted for the 2005 bombing which killed Rafik Hariri, Saad's father and a former prime minister, and 21 other people.

The trial is due to open in The Hague in the Netherlands in January.