The sentencing of Mamadi Conde, who is known as ‘Madic 100 Frontier’, comes after the president won a third term in a violently disputed election on October 18.
The 82-year-old ran after pushing through a new constitution last March which allowed him to sidestep the country’s two-term limit, provoking mass protests.
Dozens of people were killed during demonstrations, often in clashes with security forces. Hundreds were also arrested.
Mamadi Conde usually lives in Canada but is a member of Guinea’s main opposition party, the UFDG, and returned to Guinea to participate in the presidential campaign of its candidate, Cellou Dalein Diallo.
The exact charges against the activist remain unclear. However one of his lawyers, Pepe Antoine, told AFP that he had posted “very virulent” criticisms of the government on social media.
A prosecutor during his trial also said that Mamadi Conde had alleged that the government discriminated against ethnic Fulani people — a sensitive topic in the former French colony.
Guinea’s politics are mainly drawn along ethnic lines: Conde’s base is among the ethnic Malinke community while the UFDG has strong backing among the Fulani people.
Guinean courts have handed down jail terms to several other opposition activists in recent weeks, in what rights groups have described as a crackdown on dissent.
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Two opposition figures have also died in pre-trial detention in Guinea, drawing criticism from the United States and European Union.
A former opposition activist himself, Conde became Guinea’s first democratically elected president in 2010 and won re-election in 2015 before doing so again last year.
Critics accuse him of veering towards authoritarianism, however.