Opponents of the son of late President Omar Bongo, who has declared himself victor of Sunday's poll, chanted the Gabonese national anthem in a Libreville square near the heavily-guarded building of the election commission.
Rivals of Ali Ben Bongo said they feared the official results were being massaged to ensure a dynastic succession from father to son, an accusation the former defense minister denies.
"Gabon is a republic and not a kingdom. This country has decided that power cannot be passed from father to son. It's crazy," said opposition supporter Tanguy Bissanga.
Observers and financial markets have played down the risk of major instability in the central African oil nation but some unrest was expected given the dispute over the result.
Reuters witnesses said anti-riot police and gendarmes armed with tear gas launchers watched the protesters while a helicopter hovered overhead.
Land and sea borders have been closed and at least one telephone operator was blocking text messages.
Election commission officials had said they expected an announcement of the result on state television late on Wednesday but that appeared to switch to Thursday.
Ben Bongo is favorite to win in a country that has secured years of foreign investment in its oil industry but largely failed to improve the lives of most citizens.
Yet his claims of victory have been matched by rivals Andre Mba Obame, a former interior minister who resigned in July, and Pierre Mamboundou, one of the few candidates with no history of ties to Bongo.
Libreville residents said the situation across most of the city remained calm. "The situation is calmer than yesterday. Things have got better since yesterday," said Esteban Moueli.
Omar Bongo's death in June, aged 73, ended nearly 42 years of rule that brought stability to Gabon but also allegations that he lavished petrodollars on family and friends rather than using them to alleviate still widespread poverty.
Gabon hosts oil firms including France's Total and U.S.-based Vaalco, and is one of the few sub-Saharan countries to have launched a Eurobond.
Analysts say any successor to Bongo will face the challenge of diversifying the economy to replace proceeds from its dwindling oil reserves.
