Liberia's President Ellen Johnson Sirleaf is poised to win re-election in a runoff vote that has been marred by an opposition boycott and a deadly clash between police and protesters.

Former justice minister Winston Tubman, the president's challenger in Tuesday's election, has called on his supporters to boycott the poll because of alleged electoral fraud favoring Mrs. Johnson Sirleaf.

Small lines of voters were reported after the polls opened.

Violence erupted in the capital, Monrovia, Monday outside the headquarters for Mr. Tubman's Congress for Democratic Change party, where riots between Liberian police and opposition protesters killed at least one person.

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

Mr. Tubman, who has called for the election to be postponed, says Liberians have the right to abstain from voting as an expression of their displeasure with the government.

During Monday's violence, police fired tear gas at hundreds of rock-throwing supporters of Mr. Tubman.

A VOA correspondent on the scene said that at one point, U.N. 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.

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

President Johnson Sirleaf defeated Mr. Tubman in the first round, 44 to 32 percent, short of the majority required for an outright victory. The third-place finisher in the initial vote has announced support for Mrs. Johnson Sirleaf, who won the Nobel Peace prize this year for helping Liberia recover from a brutal civil war.

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