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. Sirleaf.

A VOA correspondent said voter turnout was very low, including at one polling station that had long lines during the first round of voting last month.

Security was tight Tuesday in Monrovia around the headquarters for Mr. Tubman's Congress for Democratic Change party, where riots a day earlier between Liberian police and opposition protesters killed at least two people.

Police closed three radio stations overnight, including one owned by Mr. Tubman's running mate, George Weah.

A national police spokesman told VOA that police were enforcing a court mandate to close the stations after Monday's violence.

A truck of Liberian police arrived at the CDC headquarters Tuesday, but were met by CDC supporters who came outside yelling “no police.” The police left after being asked to do so by U.N. peacekeepers who are securing the site.

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 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 backed Mrs. Sirleaf, who won the Nobel Peace prize this year for helping Liberia recover from a brutal civil war.

President Sirleaf has said Mr. Tubman's boycott violates the country's constitution and laws. Mr. Tubman had called on Liberians to abstain from voting as an expression of their displeasure. This is Liberia's second presidential election since the end of the civil war in 2003.