The decision by pro-Syrian groups led by Hezbollah caps weeks of political tension.
Amid the protests, Amr Moussa, secretary general of the Arab League, arrived in Beirut to meet Lebanese officials.
Moussa said Arab countries could not afford to be bystanders.
“The stability of Lebanon and moving towards a solution that would bring about a sure future for the country is one of our concerns,” he said.
“All of us are worried about the situation in Lebanon.”
Government supporters, who held their own mass funeral procession following the assassination of Pierre Gemayel, a member of the Lebanese majority coalition, stayed away from central Beirut on their leaders’ advice.
“When they are convinced that there is no solution except through dialogue, then welcome”
Walid Jumblatt, Druze leader
Walid Jumblatt, a member of the majority March 14 Forces alliance and de facto leader of Lebanon’s Druze Muslim community, said: “We will be steadfast, peacefully and democratically. We are here, and when they are convinced that there is no solution except through dialogue, then welcome.”
Despite the protest, the city’s annual marathon went ahead. Runners used suburban roads to skirt the demonstration.
The opposition has been demanding veto power in the government, whose majority comprises politicians from Christian, Sunni Muslim and Druze parties.
Hezbollah receives financial and logistical support from Iran and Syria, while most members of the majority March 14 Forces coalition are opposed to Syrian influence in Lebanon.
Syrian troops and security pulled out of Lebanon in April 2005 following street demonstrations in the wake of the assassination of Rafiq al-Hariri, a former Lebanese prime minister.
Politicians in the majority coalition say the opposition only wants to weaken the government and derail a UN tribunal that would try those suspected of involvement in the al-Hariri killing.
A preliminary UN inquiry has implicated Syrian and Lebanese security officials in the killing.
Six opposition ministers resigned from the cabinet last month after talks on national unity collapsed. But the depleted government approved plans for the tribunal, sparking the latest protests.