Supporters of opposition presidential candidate Alassane Ouattara celebrate his apparent election victory on a street in Gagnoa, before state television announced the re-election of incumbent President Laurent Gbagbo. (Luc Gnago/Reuters)

The streets of Ivory Coast's main city of Abidjan contained angry protests on Friday, following a stunning reversal of presidential election results.

Reuters reported that enraged demonstrators hurled concrete, set tires ablaze and ripped down billboards shortly after incumbent Laurent Gbagbo was declared the new victor.

The sudden turnaround naming Gbagbo was broadcast on national television, contradicting the previous day's results, which had opposition leader Alassane Ouattara on top.

Ouattara's victory was announced by the election chief and the results were considered credible by the U.S. and the African Union.

United Nations Secretary General Ban Ki-moon and the Security Council also officially backed Ouattara as the legitimate winning candidate in Ivory Coast's first presidential elections in a decade.

The secretary general's spokesman declared Ouattara the winner, congratulating him on Friday for the election results.

Although the vote in the West African nation was meant to restore stability after civil war erupted in 2002, Gbagbo's apparent refusal to cede power has brought about the spectre of more unrest.

Friday's latest results withdrew votes from seven voting districts considered opposition strongholds, tipping the balance in favour of Gbagbo.

The ruling party claimed the votes in those seven districts should not be counted, alleging the votes were tainted by intimidation and violence.

"The irregularities are of such a nature that they invalidate the vote [in those districts]," Constitutional Council President Paul Yao N'Dr said on state TV.

Ivory Coast is the world's top cocoa producer, but years of civil war have crippled the country's economy.

With files from the Associated Press