Liberia's President Ellen Johnson Sirleaf is set to win a second term during a runoff poll on Tuesday boycotted by her rival and marred by deadly violence.

The voter turnout appeared to be light in Monrovia and other parts of the country dominated by challenger Winston Tubman of the Congress for Democratic Change party.

The former justice minister urged his supporters to stay away from the polls because of alleged electoral fraud favoring Mrs. Sirleaf.

Security remained tight in Monrovia on Tuesday, one day after Mr. Tubman's supporters clashed with police outside the party's headquarters. At least two people were killed in the unrest.

The Liberian government closed three radio stations overnight, including one owned by the challenger's running mate.

President Sirleaf has said Mr. Tubman's boycott violates the constitution because it encourages Liberians to give up their right to vote.

Mrs. Sirleaf handily won first-round elections last month, but the Nobel Peace Prize winner did not secure enough votes to avoid a runoff.

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

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.

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