Police were called in and traffic was blocked as explosives experts examined the box.
The finds have intensified fears that a political stand-off between the government and opposition could spill over into wider violence.
Eight people were killed after violence erupted following protests and strikes led by the opposition, who have demanded a veto in government and fresh elections.
On February 13, two bombs exploded on buses on a busy commuter mountain road in a Christian area northeast of Beirut, killing three passengers and injuring 20 others.
A number of prominent Lebanese have been targeted in a series of bomb and gun attacks over the last two years.
The biggest was a suicide truck bombing that killed former premier Rafiq Hariri in February 2005.