The head of Ivory Coast's electoral commission on Thursday declared opposition leader Alassane Ouattara the winner of the West African country's contested presidential election.

Election commission chief Youssouf Bakayoko said Ouattara had won with 54.1 per cent of the vote, compared to 45.9 per cent for incumbent Laurent Gbagbo, after days of backroom wrangling during which the ruling party had physically prevented other commission members from announcing the outcome of the vote.

Bakayoko left the commission's headquarters unannounced and slipped into the fancy Golf Hotel, where he made the announcement.

The hotel then replayed his declaration on the large speakers on the hotel's lawn, blasting the news as crowds gathered and opposition supporters began running through the streets in celebration.

The race is far from over because the victory must be validated by the country's constitutional council, which is led by ruling party loyalist Paul Yao N'Dre. N'Dre appeared on state-controlled television where he said that the electoral body's results were not valid because the commission had missed a constitutionally mandated midnight deadline on Wednesday.

"Only the constitutional council is qualified to give the results of this election," he said in the broadcast. "There are some foreign TV channels amusing themselves giving results. Ivorians should consider these results null and void."

Soon after two separate decrees read on state TV announced that all foreign radio and TV broadcasts are being banned indefinitely and that the country's air, land and maritime borders had been closed.

The development heightens uncertainty and casts a shadow over the country's first election in 10 years, which was meant to restore stability after political infighting plunged it into civil war in 2002. The international community scrambled to defuse the situation, with the White House releasing a strongly-worded statement asking the parties to accept the election commission's results.

"Those provisional results have declared Alassane Ouattara the winner over incumbent President Laurent Gbagbo," the statement released late Thursday said. "Credible, accredited electoral observers have characterized the balloting as free and fair, and no party should be allowed to obstruct further the electoral process."

The conflict officially ended in 2003, but the country was severed in two, with a government-controlled south led by Gbagbo and the north, where Ouattara comes from, in rebel hands.

The continuous delay of the election by Gbagbo, whose term expired in 2005 and who claimed the election was impossible due to the war, destroyed the economy of the world's top cocoa producer. The situation sent foreign investors packing, emptying out what was once one of Africa's choicest destinations sporting skyscrapers, gourmet restaurants and lagoon-facing boulevards.

Reached by telephone Thursday, one of Gbagbo's senior advisers, Richard Assamoa, called the release of results "an attempted coup d'état."

In a news conference immediately after the announcement of his victory, the 68-year-old Ouattara called on his opponent to respect the outcome.

"I remind my brother Laurent Gbagbo of our mutual engagement to respect the results proclaimed by the independent electoral commission," he said. "I'm proud of my country, which has resolutely chosen democracy today, and I hope this leads to a durable peace in Ivory Coast."

Authorities said Thursday that police responding to a call at one of Ouattara's offices had killed four people after being fired upon.

The opposition coalition denied any weapons were on the premises and said the attackers fired first.