At least one person has died after shots were reportedly fired during an opposition protest in Monrovia ahead of Liberia's presidential run-off.
A BBC reporter saw the body of a young man who had been shot in the head.
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 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 organised 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.
The rioting broke out after thousands of CDC supporters gathered outside party headquarters to urge voters to boycott Tuesday's poll.
Police, backed by UN forces, reportedly blocked a road to prevent the CDC activists from marching through the city, before the shooting and stone-throwing broke out.
Opposition activists are said to have exchanged fire with the police, who also used tear gas.
But police spokesman George Badue said officers had not used live bullets.
He said only tear gas was used by the police "to disperse the crowd so that people who were not part of the demonstration could move about freely".
As well as the dead man, the BBC's Jonathan Paye-Layleh, in Monrovia, also saw three or four other injured people who said they had been shot.
Mr Tubman's running mate, former football star George Weah, condemned the shooting of "unarmed protesters" and called for the elections to be postponed.
President Sirleaf won the first round last month but failed to pass the 50% threshold needed for outright victory.
Mr Tubman and the CDC say there was widespread vote-rigging - charges denied by the election commission and Mrs Sirleaf's supporters.
The US, EU and African Union have all condemned the opposition's decision to pull out of the run-off.
"It's a bad signal... political leaders must be prepared to win or lose," said former Ugandan Vice-President and head of the African Union observer mission Speciosa Wadira Kazibwe, according to the AFP news agency.
Prince Johnson, a former warlord who came third in the first round, has backed Mrs Sirleaf in the run-off.
While campaigning on Sunday, Mrs Sirleaf said: "I know that nobody in this country, no matter what the talk or rhetoric, nobody really wants us to go back to war."