Gabon's main opposition leader Andre Mba Obame arrived in Libreville after 14 months in France, saying the International Criminal Court would look into a 2009 "massacre" at Port-Gentil.
Obame, leader of the now outlawed National Union (UN) party who claimed to have won the last presidential election, was greeted by up to 3,000 supporters on his return after his long absence for "health reasons".
He was not bothered by the authorities and went from the airport to his home although his followers had feared police action and even the arrest of their leader upon arrival.
He told his backers: "The ICC will soon announce that it will send an inquiry here because people were massacred at Port-Gentil. They were thrown out of helicopters."
Some 60 people were killed at the western city in 2009 after the contested election win by Ali Bongo Ondimba, Obame said, "and they (authorities) said 'it's nothing'. But the ICC is coming, the ICC is at our doors."
An official toll said three people were killed in riots after the elections, but the opposition has challenged the figures.
Obame, a former loyalist who turned to the opposition after the death of Omar Bongo in 2009, also called for a national conference to prevent further violence.
"We are calling for a sovereign national conference," he declared. "It's the only way to get the country out of the crisis without bloodshed."
The Gabonese president's office rejected the proposal, saying: "Gabon is not a country in crisis, there is no social crisis, no political crisis, no constitutional crisis. A national conference is not justified."
Obame's return is a key event in Gabon, which has not had a main opposition figure since the death of Pierre Mamboundou, leader of the UPG (Union of the Gabonese People), in October 2011.
Once close to Ali Bongo's father Omar Bongo Ondimba, Obame switched sides after the older Bongo's death in 2009 to form the opposition to the late president's son.
Obame's party was dissolved by authorities in 2011 after he proclaimed himself president in a 2009 election won by Bongo.
Accused of "disturbing public order" and "threatening state authority", for which he risks a prison sentence of between two months and one year, and stripped of his parliamentary immunity, Obame left Gabon for France.
Authorities deny that an increased police presence in the capital in the last few days is linked to the politician's return, which they called a "non-event".
"You cannot reproach state authorities for not maintaining the security of Gabonese and then complain when police are out on intersections," said presidential spokesman Alain Claude Bilie-By-Nze.
Though officially dissolved, the National Union party continues to function and supporters prepared for Obame's return, timed for just a few days before Gabon's independence day on August 17.
In recent days, minibuses have brought people from the province of Woleu-Ntem in the Fang heartlands -- the ethnic group to which Obame belongs -- and hundreds of posters were put up around the capital.
The National Union and other opposition groups have demanded a national conference to call for sweeping reforms including the dissolution of Gabon's national assembly and a new constitution. They also want elections in 2013.
In returning Obame risks imprisonment if authorities decide to pursue him.
"Mr Mba Obame himself said he was ready to take up his work again. So he is ready to answer a call to justice," said Bilie-By-Nze.
Last month a combative Obame said he was ready for the fight.
"I left Gabon in a wheelchair, I'll come back on my two legs. People that have said I'm dead and gone had better prepare to fight my ghost."