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.

The unrest Monday 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.

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

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.