Opposition supporters began to gather Sunday for a mass rally marking the sixth anniversary of a popular uprising against Syrian troops in Lebanon, demanding the disarming of Hezbollah radicals.

Thousands of Lebanese headed to Martyrs' Square in central Beirut early Sunday, waving Lebanese flags and the flags of the country's Christian and Sunni Muslim pro-Western political parties, AFP correspondents said.

Convoys could also be seen across the country heading to the capital, blaring songs and displaying pictures in support of slain ex-premier Rafiq Hariri.

The anniversary follows a drawn-out political crisis which saw the Iranian- and Syrian-backed Hezbollah topple Saad Hariri's unity government in January, capping a long-running feud over a UN-backed Special Tribunal for Lebanon.

The tribunal -- tasked with investigating the 14 February 2005 murder of Rafiq Hariri, father of the outgoing prime minister, and 22 others -- is reportedly readying to implicate members of Hezbollah in the killing.

Sunni Muslim billionaire Najib Mikati, appointed with Hezbollah's backing, has been tapped to succeed Saad Hariri and has since January 25 sought to form a government.

The Hariri-led opposition has announced it will sit out of Mikati's government, which it accuses of being "Hezbollah's cabinet".

Rafiq Hariri's assassination by a huge car bomb saw the rise of a US- and Saudi-backed alliance that became known as March 14, named after a day of massive anti-Syrian protests dubbed the "Cedar Revolution."

Combined with international pressure, the protests in the weeks after the killing led to the pullout of Syrian troops from the eastern Mediterranean country in April 2005 following a 29-year deployment.

Red billboards urging supporters of the Hariri camp to head downtown on Sunday lined highways across the capital, bearing slogans such as "NO to assassinations," "NO to oppression" and "NO to the rule of arms."

Other billboards, which no party has yet claimed responsibility for, have surfaced in the capital, reading: "Israel too wants to topple arms," a reference to Hezbollah's arsenal.

The opposition has accused Hezbollah, the only party not to have turned in its arms after the 1975-1990 civil war, of having used its arsenal to intimidate MPs into voting against Hariri's re-appointment after his unity cabinet collapsed.