BEIRUT, Dec. 1 (Xinhua) -- Lebanese police tightened security on Friday to prevent any riot in imminent street demonstrations as thousands of pro-Syrian supporters began gathering in downtown Beirut to topple the current government, local press reported.

    Thousands of protestors waving Lebanese national flags gathered in downtown Beirut and chanted slogans to call for the government's stepping down, Hezbollah's al-Manar TV reported.

    Large numbers of security forces, backed by armored personnel carriers, deployed in central Beirut, the footage broadcasted on al-Manar TV showed.

    The heavy security came amid fears that the protests could turn into street clashes between pro-Syrian and anti-Syrian groups and Hezbollah supporters could try to storm Prime Minister Fouad Seniora's offices.

    Hundreds of heavily armed police and combat troops unfurled barbed wire and erected barricades around Seniora's office. The forces also deployed at the capital's entrances.

    The Lebanese opposition Thursday called on the public to take to the streets for a mass demonstration to demand a new national unity government.

    Mass street demonstrations are set to officially begin at 3 p.m.(1300 GMT) and it will be followed by an open-ended sit-in against Seniora's government, led by an anti-Syrian parliament majority.

    Lebanese Shiite group Hezbollah and pro-Syrian allies have earlier mobilized for mass street protests to topple incumbent government after the cabinet on Nov. 13 approved with absence of six pro-Syrian ministers a UN draft document for the creation of an international tribunal on the case of former Prime Minister Rafik Hariri's killing.