Protesters targeted facilities owned by French oil giant Total and U.S. oil field services firm Schlumberger in the Port Gentil oil hub, and ex-colonial power France's consulate there, the French Foreign Ministry said.
The government later imposed a curfew on the city, and said extra security measures would be implemented across the country, without giving details of what those measures would be, nor when the Port Gentil curfew would be lifted.
A Reuters witness touring the capital Libreville after anti-riot police had dispersed protesters said its streets were largely deserted, in what could prove an early sign that former defence minister Ben Bongo was asserting his authority.
"I want to be president of all the Gabonese," Ben Bongo, 50, declared on his family television network TeleAfrica after the interior minister declared him victor of Sunday's poll with 47.1 percent of the vote.
French Secretary of State for Cooperation Alain Joyandet said election observers had seen only minor irregularities.
"Observers who have given us their accounts have all said the election took place in acceptable conditions," he told RTL radio. "If the losing candidates want to contest the result, they should do so in the constitutional court," he said.
Ex-interior minister Andre Mba Obame scored 25.9 percent and Pierre Mamboundou, one of the few main Gabonese politicians with no ties to the Bongo family, scored 25.2 percent. Both had also declared victory before the results were known and immediately said they rejected the outcome.
"We condemn these results. It is a constitutional coup d'etat," said Richard Mombo, secretary general of Mamboundou's Union of Gabonese People (UPG) party.
Mombo told Reuters by telephone that Mamboundou had been "seriously injured" in clashes with security forces in the capital earlier but gave no details of his condition.
Dwindling Oil
Ben Bongo's rivals accuse him of rigging the result to ensure a dynastic transfer of power from his father, who brought stability during nearly 42 years of rule but faced accusations he used petrodollars to enrich family and friends.
In Port Gentil, an unnamed resident was quoted by French state radio France Info as saying prisoners freed from a jail were roaming the streets and had set fire to a petrol station.
"There are thousands of people. There is the opposition but it is not just the opposition. Everyone is taking advantage of this, not just the prisoners," it quoted him as saying.
U.N. Secretary-General Ban Ki-moon called on political leaders to show restraint, and for their supporters to use legal channels to resolve grievances.
The dispute comes after polls this year in Niger, Congo Republic and elsewhere in the region where incumbent leaders have been accused of rigging results to stay in power.
Observers and financial markets have played down the risk of major instability in Gabon—a rare sub-Saharan country to have a Eurobond—but some unrest was expected given the dispute over the result.
"The result is not unexpected, Ali Ben Bongo had seemed to be groomed by his father, but there has been a groundswell of opposition," said Kissy Agyeman-Togobo of IHS Global Insight. "I think the words 'under control' are accurate," said one Western diplomat in Gabon.
"People are hunkering down. Gabon doesn't have a history of violence and I think that is being reflected. It will take a little time ... but it will get back to normal."
Little change was expected in the business-friendly policies espoused by the late president, whose son now faces the challenge of replacing Gabon's dwindling oil reserves with alternative revenues.
"In the longer term, Gabon needs to accelerate the reforms it has just started if it wants to attract greater levels of investment and diversify its economy," said Michael Hugman at Standard Bank.
