The incident occurred as a convoy of supporters from the Progressive Socialist Party of Druze leader Walid Jumblatt was driving past the local headquarters of the rival Lebanese Democratic Party in Aley, about 15 kilometres (eight miles) east of Beirut, the official told AFP.

"Shots were heard and a passer-by was wounded by a stray bullet," he said, describing his condition as serious. He said a girl who was travelling in the party convoy was slightly injured.

An official from Jumblatt's party -- which is part of the ruling anti-Syrian coalition -- told AFP that four people were injured when the convoy came under fire.

But a statement from the Democratic Party said its Aley headquarters had come under fire from PSP "militia members", triggering an armed confrontation. It warned that the incident could spark civil strife among the Druze population and said Jumblatt would be held responsible.

Several similar incidents have occurred recently in Beirut, with tensions running high because of a long-running political crisis pitting the ruling coalition against the Syrian-backed opposition.

Jumblatt had earlier launched an outspoken assault on the opposition, warning his side was ready for war.

"You want disorder? It will be welcomed. You want war? It will be welcomed. We have no problem with weapons, no problem with missiles. We will take them from you," Jumblatt told a news conference.

Shots were also fired on Sunday in an altercation between supporters of parliamentary majority leader Saad Hariri and speaker Nabih Berri's security services in Beirut, the security official said.

"A convoy from the Future movement was driving by Berri's residence. Apparently some heated words were exchanged with Berri's security service and shots were fired."

He said there did not appear to be any injuries.

Sunday's clashes come just days before Lebanon marks the third anniversary of the assassination of Hariri's father Rafiq Hariri in a massive Beirut bomb blast on February 14, 2005.

"It is very strange to see that the trend is to escalate the situation at a time when the Arab League is trying to resolve the standoff over the presidency," said one official close to the opposition.

Saad Hariri's Future movement, which leads the ruling coalition, is at loggerheads with the opposition -- of which Berri is a leading member -- over the choice of a new president.

Lebanon has been without a head of state since November when the pro-Syrian Emile Lahoud stood down and the rival factions have been unable to agree on a successor, with 14 planned parliamentary votes cancelled because of the deadlock.