"Francois Bozize has definitely been in the capital of the CAR since yesterday. These aren't just rumours, it's true," said Bertin Bea, the secretary general of the Kwa na Kwa party.
Bea, speaking at a press conference held at party headquarters, promised: "In the coming days, Francois Bozize will address his fellow countrymen."
Government spokesman Maxime-Ange Kazagui dismissed the claim, saying, "This announcement doesn't seem credible to me."
Bozize faces an international arrest warrant, initiated by the CAR in 2013, for crimes against humanity and incitement to genocide.
A former armed forces chief, Bozize seized power in CAR, one of the world's poorest and most unstable countries, in March 2003.
He held office for a decade before being overthrown by a coalition of mainly Muslim armed groups, the Seleka.
His lawyers and supporters have persistently campaigned for the government to allow him to return.
Monday's claim came five days after an court in Bangui rejected a request that a 2016 ban on airlines taking him back to the country be suspended.
Bea, in other comments, said, "Do not ask me how he arrived" back in the CAR.
After Bozize fled, Seleka leader Michel Djotodia proclaimed himself president of the mainly Christian country.
Fierce fighting erupted between predominantly Christian and Muslim militia, prompting intervention by the France, the former colonial power, under a UN mandate.
A UN force, MINUSCU was deployed and presidential elections were held, with victory for former prime minister Faustin-Archange Touadera.
In February this year, the country made its eighth attempt at sealing a lasting peace, with an agreement between Touadera's government and 14 rebel groups who control most of the country's territory.
Nearly a quarter of the country's 4.6 million people have fled their homes.
The CAR is ranked next-to-last, after Niger, on the 2018 UNDP's Human Development Index, which compares longevity, education, and income per capita. Life expectancy is just 52.9 years.
On 2014, Bozize was placed on a UN sanctions list for "committing or supporting acts" that undermined peace and stability -- a reference to Bozize's support for Christian militias in 2013.