Liberia's top opposition figure said on Saturday his party was seeking legal options to have President Ellen Johnson-Sirleaf's re-election in a November 8 run-off annulled, calling the vote a political farce of the highest order.

Johnson-Sirleaf, a newly named Nobel peace laureate and currently Africa's only elected female head of state, won the poll with over 90 percent of the ballots cast, but the election was marred by an opposition boycott and electoral violence.

Our lawyers are busy working on all of our legal options and we have several, opposition leader Winston Tubman, Johnson-Sirleaf's main presidential challenger, told a news conference at his party headquarters in Monrovia.

We believe that everything that flows from Tuesday's elections must be annulled and a new round of elections scheduled within a month.

Tubman had urged his supporters to boycott the poll over alleged fraud in the first round, in which he won about 33 percent of the vote to Johnson-Sirleaf's 44 percent. He had said he was seeking changes to the vote counting process for the run-off, and a delay to give him more time for campaigning.

The day before the vote, security forces used tear gas and live ammunition to disperse a crowd of opposition supporters in and around the party headquarters, killing two. Tubman was in the headquarters at the time and said he felt he was personally targeted, a charge the government denies.

Johnson-Sirleaf told Reuters on Friday that she would work with Tubman only if he accepted the results of the run-off, but would otherwise seek reconciliation with the country's opposition political parties.

The election was the West African state's first locally organised presidential contest since a civil war ended in 2003, and was meant to pave the way for billions of dollars of international investment in its resource industries.

International observers said they saw no evidence of fraud during the election process, but regretted the boycott and the electoral violence.

Liberia is one of the world's poorest countries and is struggling to rebuild after the 14 years of fighting left its infrastructure and economy in ruins.

(Reporting by Richard Valdmanis, Alphonso Toweh, and Clair MacDougalll; editing by Tim Pearce)