Suleiman Franjieh, a Christian opposition leader, told Hezbollah's Al-Manar television the next steps "will be nothing compared to what we saw today" if the government does not respond to the opposition's demands.
The opposition has become increasingly frustrated in its efforts for political change.
Two months of sit-in protests outside the central Beirut offices of Fouad Siniora, the Lebanese prime minister, have failed to force him to step down or form a new government giving the opposition the extra power it is demanding.
Siniora has vowed not to give in, saying in a televised address on Tuesday: "We will stand together against intimidation and to confront sedition."
But he repeated his willingness to discuss a political solution to the impasse and called for a special session of parliament.
Nicholas Burns, the US undersecretary of state, speaking in Dubai, said the international community must support the Siniora government against "those who would destabilise it".
Siniora has left Beirut to attend a conference of donor nations in Paris being held on Thursday, aimed at raising billions of dollars in aid for rebuilding Lebanon after the devastation wreaked by last summer's Israel-Hezbollah war.
Lebanon's security forces struggled to contain Tuesday's violence.
In the Christian Batroun region in the north of the country, government supporters and opponents fired on each other, killing a pro-Siniora protester.
Across Lebanon, 44 people sustained gunshot wounds and about 80 others were injured in fistfights or stone-throwing attacks.
Al Jazeera's Arabic Channel reported six people had died in the confrontations, three opposition supporters and three government supporters.
Many businesses closed as workers stayed home, either in support of the strike or because of blocked roads.
Some schools closed because of the unrest, others opened but quickly sent pupils home.