At least one person has died after shots were reportedly fired during an opposition protest in Monrovia ahead of Liberia's presidential run-off.
Congress for Democratic Change (CDC) candidate Winston Tubman has pulled out of Tuesday's vote, alleging fraud.
Nobel Peace laureate Ellen Johnson-Sirleaf, Africa's Africa's first elected female head of state, is running for another term.
She was first elected after Liberia's first post-war election in 2005.
These are the first elections organized by Liberians since the 14-year conflict ended. The previous ones were run by the large UN peacekeeping mission.
Justice Minister Christiana Tah told the BBC that security would be stepped up for the elections following the violence and that an investigation would be opened.
She could not confirm the number of casualties. Some CDC officials say four people died.