A decision by Africa's longest-serving leader to seek another seven years in power has been criticised.
Bongo has ruled Gabon since 1967
President Omar Bongo, who came to power in 1967, said at the weekend that he had "listened to the people" and would seek another term in December's poll.
The 69-year-old's government recently told opposition leaders they would lose their passports if they left Gabon.
A group of exiles, called Bongo Must Leave, says Gabon's people have had enough of his authoritarian rule.
"We are against the whole system, which Mr Bongo represents. We want to clean it all up," leader Serge Besac told the BBC's Network Africa by telephone from Washington.
The 69-year-old president launched a crackdown after complaints by an opposition leader that the electoral register for December's presidential poll had been inflated.
He was also said to be angered by a protest organised outside the United Nations last month, staged as he was addressing a summit.
The government in Gabon has made changes to the constitution which would allow President Bongo to run for office as many times as he wishes.