Security sources said the argument took place between supporters of Parliament Speaker Nabih Berri's Amal movement and the Partisans of the Future movement led by Saad Hariri over the hanging of a picture of Shiite Imam Musa al Sadr who went missing in the 1970s during a trip to Libya.

Lebanese army troops promptly set up checkpoints and began patrolling the area to avoid further clashes.

The incident came a few hours after unknown assailants hurled a sonic bomb in Beirut's Tayyouneh tunnel, causing damage but no casualties.

In the nearby Muslim neighbourhood of Basta in the capital a similar bomb was thrown but failed to explode.

For the past ten days, several neighbourhoods of Beirut have been witnessing sporadic clashes between Sunni followers of the Western- backed ruling majority and others loyal to the Hezbollah-led opposition, raising concerns that the situation is heading towards clashes similar to those in May.

The clashes which killed more than 82 people ended after Lebanese rivals leaders reached an agreement in Doha, Qatar to settle their differences inside the government institutions and not in the streets.

The accord led to the election of Lebanese President Michel Suleiman and the formation of a national unity government in which Hezbollah and its allies have the veto power.

Meanwhile a UN report said Lebanon's eastern borders remain open to the smuggling of weapons from Syria, Lebanese daily an-Nahar reported.

The report was prepared by an independent committee assessing the situation along the Lebanon-Syria border.

UN Security Council Resolution 1701 which ended a 33-day war between the Hezbollah militant movement and Israel in the summer of 2006 banned the illegal transfer of weapons into Lebanon.

The report published in New York criticized both Lebanon and Syria for the lack of progress in preventing "smuggling in general, and the smuggling of weapons in particular, despite all pledges made by both countries." (dpa)