The demonstrators, led by the opposition group Hezbollah, have blocked most of the roads surrounding the government headquarters of Prime Minister Fouad Siniora.
Hezbollah leader Hassan Nasrallah had called for a massive turnout against what he said was an 'incapable government'.
Hezbollah, bolstered by its strong resistance during the 34-day war with Israel, is demanding a greater share in government that would give it power to veto legislation.
However, anti-Syrian politicians believe the opposition demands are a ploy to avoid the creation of an international tribunal to try suspects for the murder of former Prime Minister Rafik al-Hariri.
Parliamentary majority leader and son of the former leader, Saad al-Hariri, has insisted that the government will not fall due to 'pressure from the street'.
This show of strength by the opposition follows last week's funeral for murdered anti-Syrian industry minister Pierre Gemayel which brought hundreds of thousands of government supporters on the streets.
Mr Gemayel was the sixth anti-Syria figure to be assassinated in Lebanon over the past two years.
Support for Siniora
Mr Siniora's government has received strong backing from foreign states, including Jordan and Saudi Arabia.
US State Department spokesman Tom Casey said Washington denounced the 'threats of intimidation or violence' which he said were aimed at toppling Lebanon's legitimate and democratically elected government.
Mr Casey also accused Syria and Iran of instigating the show of force in order to 'destabilise Lebanon'.
