A man was killed Monday in Monrovia when an opposition rally turned violent on the eve of a run-off vote which President Ellen Johnson Sirleaf's challenger accuses the Nobel winner of rigging.

Shooting erupted after anti-riot police and UN peacekeepers using water cannons tried to block an unauthorised march by thousands of demonstrators supporting their candidate Winston Tubman.

Tubman's call to boycott the second of the west African country's second post-war polls on Tuesday drew wide international condemnation and raised fears that Liberia was being dragged back to the dark days of civil war.

The body of a man aged around 20 with a gunshot wound to the head was seen by journalists in the offices of Tubman's Congress for Democratic Change headquarters, outside which protesters clashed with police.

One policeman at the scene said a protester fired the first shot but an eye witness blamed the security forces for the flare-up and the man's death.

"He was standing in front of the building when a policeman shot and I saw him going down," said witness Anita Mulba.

A Liberian journalist reported seeing two more bodies at a police station not far from CDC headquarters.

Sirens wailed throughout the city, as police attempted to disperse the protesters by firing tear gas. The UN Mission in Liberia's helicopters circled and its peacekeeping troops were out in force alongside riot police.

Tension rose when riot police blocked the road after protesters attempted to stop advancing UNMIL water cannons.

"You will see UNMIL staff, police and military on the streets and in the air all around the country," said UN special representative in Liberia Ellen Margrethe Loej on UNMIL radio.

"We are here to ensure everything is peaceful and we are here to deter anyone who intends to destroy the peace."

The protesters, who chanted slogans such as "we want justice, we want freedom", planned a march later in the day which had not been officially approved.

"As long as they have not obtained permission from the ministry of justice the police will not allow them," said police spokesman George Badoo.

Tubman, a 70-year-old Harvard-trained lawyer, set the nation on edge with his call to boycott an election seen as a test of Liberia's fragile democracy eight years after a long and brutal conflict that left some 250,000 dead ended.

He was confident of a first round victory, mixing his education and experience with the crowd-pleasing former football star George Weah as his running mate, but cried foul after losing to Sirleaf by more than 10 percentage points.

Tubman made several demands to the National Electoral Commission, securing the resignation of its chairman, but said he was still not convinced the process would be transparent and would not accept the outcome.

"I am in support of the boycott, our demands were not met. At times you have to do unconstitutional things for a principle in your society," Kareem Marshall, a young public administration student, said before Monday's rally.

A statement published in local newspapers by the National Electoral Commission urged the nation's 1.8 million voters to cast their ballots "without engaging in violence or confrontation."

"This election is a crucial decider of how we as a nation and as a people will continue to move towards peace, democracy and progress," it said.

The international community has condemned Tubman's boycott call after some 800 foreign observers said the October 11 poll was free and fair.

EU foreign policy chief Catherine Ashton said in a statement she "regrets the opposition's decision not to participate."

"This action deprives Liberians of the choice they deserve and weakens rather than strengthens the democratic process," she added.

The United States said it was "deeply disappointed", calling Tubman's claims of fraud "unsubstantiated."

"We are very concerned. It's a bad signal ... political leaders must be prepared to win or lose," the head of the African Union observer mission, Speciosa Wadira Kazibwe, a former Ugandan vice-president, told reporters.

Sirleaf, hailed abroad for her role in restoring the nation still heavily reliant on an 8,000-strong UN peacekeeping mission, but criticised at home, says she wants a second term to rebuild the "broken country."

Having won 43.9 percent in the first round, she is poised for re-election after winning the support of key smaller parties such as that of Prince Johnson, a notorious former warlord who was filmed ordering the torture of military dictator Samuel Doe.

"I know that nobody in this country, no matter what the talk or rhetoric, nobody really wants us to go back to war," she said while campaigning Sunday.