Gabon's constitutional court has upheld President Ali Bongo's election victory, rejecting opposition calls for a recount.
There have been fears that such a decision could trigger fresh violence.
The government said it would hold opposition leader Jean Ping responsible if clashes erupted following the ruling.
President Bongo won August's election by just 6,000 votes but the opposition says the poll was rigged.
Following the court ruling, President Bongo called for a "political dialogue" with the opposition.
Correspondents say residents of Libreville were stockpiling food ahead of the court ruling. There were long queues at banks and supermarkets on Friday and the French embassy told its citizens to stay indoors.
Riot police have been deployed across the city.
Mr Ping had alleged fraud in one of the president's main strongholds, Haut-Ogooue province, where Mr Bongo won 95% of the vote on a turnout of 99.9%.
Mr Ping had warned that Gabon could face serious instability if the court rejected his appeal for a recount in that province.
The court partially changed the results of the bitterly-fought election, giving President Bongo 50.66% of the vote and Mr Ping 47.24%.
The result of the election on 27 August sparked days of deadly violence.