Riots have erupted between Liberian police and opposition protesters in Monrovia, killing at least one person on the eve of a presidential runoff vote.

Police fired tear gas at hundreds of rock-throwing supporters of former justice minister Winston Tubman, the incumbent president's challenger in the runoff.

Tubman has called on his supporters to boycott Tuesday's poll because of alleged electoral fraud favoring President Ellen Johnson-Sirleaf.

Liberia's government says the polls will go ahead as scheduled despite the violence Monday which erupted outside the headquarters of Mr. Tubman's Congress for Democratic Change party.

A VOA correspondent on the scene said that at one point, United Nations peacekeepers charged with securing the scene took defensive positions against Liberian riot police.

Witnesses said guns were fired, but it was not clear who may have fired them.

Members of Mr. Tubman's party say several other people were killed in the clashes, but this could not be confirmed.

U.S. President Barack Obama said the international community will hold accountable those who choose to obstruct Liberia's democratic process. He also encouraged security forces in Liberia to exercise maximum restraint and allow peaceful protests.

President Sirleaf has said Mr. Tubman's boycott violates the country's constitution and laws, and imperils Liberia's future.

Mr. Tubman says Liberians have the right to abstain from voting as an expression of their displeasure with the government.

Election observers said the first round of voting was generally free and fair.

President Sirleaf defeated Mr. Tubman in the first round, 44 to 32 percent, short of the majority required for an outright victory.

This is Liberia's second presidential poll since the end of a civil war in 2003.