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 erupted in Monrovia on Monday outside the headquarters of Mr. Tubman's Congress for Democratic Change party.

A VOA correspondent on the scene said the situation remained very tense as protesters continued to confront police and United Nations peacekeepers on the scene.

Security forces also circled the executive mansion and foreign ministry building.

There are reports that gunfire was exchanged, but this could not be immediately confirmed.

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.