Liberian police are reported to have stormed the headquarters of the presidential challenger Winston Tubman, but were pushed back by UN peacekeepers.
Earlier, at least three people died in the capital Monrovia after riot police tried to break up a crowd of opposition supporters.
There were conflicting reports as to how the violence had started. The opposition CDC party is boycotting Tuesday's presidential run-off election.
Al Jazeera's Yvonne Ndege is in Monrovia.