Soldiers and riot police patrolled Libreville's main thoroughfares and reinforced the guard around the presidential palace, an AFP reporter said.
Traffic was lighter than usual in Libreville on Monday as many shops and businesses remained closed amid growing tension over the uncertain outcome of the election, the preliminary results of which will be announced Wednesday.
But former defence minister Ali Bongo, long-time opposition leader Pierre Mamboundou and the ex-interior minister Andre Mba Obame have all insisted they won most votes in Sunday's election.
As the son of Omar Bongo Ondimba -- who ruled the former French colony for 41 years before his death in June -- Ali Bongo was seen as the favourite to take power in the polls. His party proclaimed him as victor without waiting for the vote count to be completed.
"Information received from different constituencies in Gabon and abroad put me largely as a winner," Bongo told journalists Monday. "I'm waiting for the competent authorities to announce the results officially."
There were similarly bullish pronouncements from Mamboundou who has already begun talking about his government's economic plan, and from Mba Obame who said the voters had shown "a strong desire to make a break" from the past.
"It will take a miracle to stop us," Mba Obame told AFP.
Louis Gaston Mayila, of the five-party coalition that backed Mamboundou, claimed victory and warned that "If there's an electoral hold-up, I'd fear for the people's reaction."
Mba Obame, a former defence minister, said the build-up of troops around the capital was unwarranted.
"It looks like a coup," he told journalists.
"What could justify security forces when everyone, including France, recognises that the election took place under good conditions? Nothing. This is provocation," he added.
Gabon's interim president Rose Francine Rogombe called for calm and exhorted the candidates to accept the result.
"Democracy is about accepting success and defeat," she said.
Former colonial power France noted what it called "the good conduct" of the election, despite what it said had been some technical difficulties.
Earlier Monday, one official from the vote commission, who asked not to be named said some remote districts had not yet sent their results to the capital, thus dampening prospects of a swift announcement.
Late Monday, the head of the electoral commission, Rene Aboghe Ella, said that results would be announced on Wednesday between 6:00 pm (1700 GMT) and midnight.
Ahead of the vote, many candidates questioned the reliability of the electoral roll, saying 813,164 eligible voters in a country of 1.5 million was way too high and suggested fraud.
Gabon is sub-Saharan Africa's fourth biggest oil producer, the world's third biggest provider of manganese -- a metal with industrial metal alloy uses, particularly in stainless steels -- and Africa's second biggest wood exporter.
But an estimated 60 percent of the population live below the poverty line.
Though many of the 18 candidates have decried the lack of development in the west African nation, several of them served in the government for years.