AFP news agency quotes a judicial sources as saying 23 people were arrested after Thursday's march.
A number of people were injured, the unnamed source added.
President Bongo, Africa's longest-serving leader, has ruled oil-rich Gabon in 1967.
He has now won three elections since a multi-party system was introduced in 1991.
The incumbent stood on his record of maintaining a stable Gabon in an unstable region.
But his opponents accuse him of using the nation's wealth to buy votes.