Liberia's main opposition party has threatened to boycott next month's presidential run-off poll unless the elections commission chief is replaced.

The party of former justice minister Winston Tubman on Tuesday accused the electoral body of rigging the first round of voting October 11.

An official with Mr. Tubman's Congress for Democratic Change says electoral commission chairman James Fromayan is a Sirleaf supporter.

The official, Jeremiah Robertson, says the party will not take part in the November 8 runoff unless Fromayan is replaced.

The electoral commission is expected to formally announce Tuesday that Mr. Tubman placed second, well behind incumbent President Ellen Johnson Sirleaf.

However, nearly-complete election results show Mrs. Sirleaf did not get the majority of votes needed to avoid a run-off.

President Sirleaf said the rule of law will determine what happens in the second round of voting, not political demands.

Observers from the Carter Center and the Economic Community of West African States have said the first round of voting was largely free and fair.