Tensions are high in Liberia as the country's presidential rivals are trading accusations of voter intimidation, days before a scheduled run-off election.

Former justice minister Winston Tubman is calling on Liberians to boycott the Tuesday run-off election where he faces President Ellen Johnson Sirleaf. He alleges that corruption in the electoral commission means the vote will not be fair.

In a nationwide address, President Sirleaf accused Tubman of violating the constitution by encouraging Liberians to give up their right to vote. The president says Tubman is dropping out of the run-off because he knows he will lose.

The United States said Saturday it was disappointed by Tubman's call to boycott the election, saying claims of a fraudulent voting in the October 11 election were unsubstantiated.

President Sirleaf won more than 40 percent of the vote in the first round.

Tubman, who received more than 30 percent, alleged voting irregularities because electoral officials changed vote totals.

Election observers from the Carter Center and the Economic Community of West African States say problems in the first round did not affect the overall outcome.