Provisional results show a challenger has won Sao Tome and Principe's presidential runoff election after President Manuel Pinto da Costa boycotted the vote.

The West African island nation's National Electoral Commission said on its website Monday that former prime minister Evaristo Carvalho collected just over 42,000 votes from the former Portuguese colony's about 111,000 registered voters.

The president had dropped out of the race, alleging the ballot was rigged and listing irregularities in a letter to the Supreme Court

Carvalho, who lost to da Costa in the 2011 presidential election, is the deputy leader of Independent Democratic Action party, which runs the government and has a majority in Parliament.