Omar Bongo embodied the close ties between France and its former colonies
Gabon's government has denied French media reports that President Omar Bongo is dead.
Prime Minister Jean Eyeghe Ndong said 73-year-old Mr Bongo, who reportedly has cancer, was "alive and well".
The government in Spain - where Africa's longest-serving leader was being treated in a clinic - has also insisted he is alive.
Mr Bongo has led Gabon since 1967, but stopped work in May. He faces a French inquiry into corruption allegations.
The BBC's Linel Kwatsi in the capital Libreville says the city appears calm and as normal - however some people have been stockpiling food in case shops close if the president's death is confirmed.
The government of the oil-rich nation has asked people not to panic.
The death of the Gabonese veteran leader was reported by AFP, which quoted a French government source, and also by the website of French magazine Le Point, quoting a source close to Mr Bongo's entourage.
Gabon's government has maintained close economic and political ties to France, its former colonial power, since independence.
Led Gabon for 42 years
Kept close economic and political ties with France
Oil money means Gabon officially one of richest countries in Africa
Reported to have cancer
His son is defence minister
His daughter is his chief of staff
He denies corruption charges in French courts
Introduced multi-party elections in 1993 - opposition complains they are not fair
A statement from the Gabonese presidency said:
"The presidency of the Gabonese Republic would like to stress that the President of the Republic, the Head of State, His Excellency Omar Bongo is not dead. He is continuing his holiday in Spain following his check-up at the Quiron Clinic in Barcelona."
"The family has been visiting him this morning," Mr Ndong told reporters on Monday.
The premier, speaking at the clinic where Mr Bongo is being treated, reportedly for cancer, criticised media reports of the veteran leader's death.
Earlier, the prime minister had responded to the reports by telling Gabonese TV: "If such a situation comes about, I would think that the president's family would naturally get in touch with me."
The Spanish foreign affairs ministry backed Mr Ndong's assertion, saying: "We have confirmed that he [President Bongo] is alive. We have no further information about him."
But later on Monday, reports in the Spanish media said Mr Bongo had died shortly after Mr Ndong's news conference. They quoted members of Mr Bongo's entourage as saying the African leader had died at 1200GMT.
The clinic and the Spanish government refused to comment on the latest reports.
Mr Bongo became vice-president in 1967, taking over as head of state later that year after the death of Gabon's first post-independence President, Leon Mba.
Oil earnings mean that Gabon, with a population of just 1.4 million, is officially one of Africa's richest states but analysts say that the political elite keep most of the money for themselves.
Mr Bongo is one of three African leaders being investigated for alleged embezzlement by a French judge - the others are Denis Sassou-Nguesso of the Republic of Congo and Teodoro Obiang Nguema of Equatorial Guinea.
It is alleged that the properties owned by Mr Bongo's family in France could not have been purchased with official salaries alone.
Mr Bongo denied any wrongdoing.
Analyst say he has built a powerful dynasty during his years in office.
Opposition leaders have claimed his son, Ali-Ben Bongo, currently defence minister, is being manoeuvred to take over.
Born Albert Bernard Bongo, he converted to Islam in 1973 and changed his name to El Hadj Omar Bongo.
His wife, Edith Lucie Bongo, President Sassou-Nguesso's daughter, died in March 2009.