Gabon soldiers on state radio early Monday called on the people to "rise up" and announced a "national restoration council" would be formed, as an ailing President Ali Bongo is out of the country.
Shots were heard in the around state television headquarters in the centre of the capital at about the same time as the message was read at 6:30 am (0530 GMT).
Military vehicles blocked access to the boulevard where the offices are located, an AFP correspondent said.
The message was read on state radio by a person who identified himself as the deputy commander of the Republican Guard and head of a group called the Patriotic Youth Movement of the Gabonese Defence and Security Forces.
Three soldiers wearing the green beret of the Republican Guard were visible on a video of the speech circulating on social media and authenticated by AFP.
The movement "calls on all young people from forces for the defence and security and Gabonese young people to join us," the officer said, announcing that a "national restoration council" would be formed.
"We cannot abandon our homeland," he said.
"The eagerly awaited day has arrived when the army has decided to put itself on the side of the people in order to save Gabon from chaos.
"If you are eating, stop; if you are having a drink, stop; if you are sleeping, wake up. Wake up your neighbours... rise up as one and take control of the street," he said on the radio.
He called on people to occupy public buildings and airports throughout the country.
The developments came as Bongo is recovering at a private residence in the Moroccan capital Rabat.
The 59-year-old leader has not been back to Gabon since he fell ill in Saudi Arabia on October 24.
It was revealed last month that he had suffered a stroke.
On December 31, he addressed the country for the first time since falling ill, saying in a recorded speech from Morocco that he had "been through a difficult period."
The Bongo family has governed the oil-rich equatorial African nation for five decades.
Ali Bongo was elected head of state after his father's death in 2009.
He was narrowly re-elected in 2016 following a presidential poll marred by deadly violence and allegations of fraud.
Libreville, Gabon | AFP