The military court in Yaounde found radio journalist Mancho 'BBC' Bibixy guilty of "acts of terrorism, hostility against the homeland, secession, revolution and insurrection", an AFP reporter said.
Bibixy was among several activists arrested last January after anti-government protests erupted in the two main English-speaking provinces, known as the Northwest and the Southwest Regions.
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The anglophone minority - which dates back to the colonial period and makes up about a fifth of Cameroon's population of 22 million - has long complained about suffering discrimination at the hands of the French-speaking government.
Bibixy is one of the voices associated with a radical movement in the anglophone town of Bamenda in the Northwest.
Five other anglophones on trial alongside the journalist received jail terms ranging from 10 to 15 years on Friday, while one other person was acquitted.
The six convicted were also ordered to pay a joint fine of 268 million CFA francs (398,000 euros) and must also pay the legal fees.
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"The decision is excessive and pointless because it will not resolve the problem" of Cameroon's anglophone crisis, said Claude Assira, one of the lawyers representing the accused. He told AFP he would appeal the decision.
The conflict spiraled after President Paul Biya, in power for more than 35 years, rejected demands for greater autonomy.
His refusal prompted separatists to declare independence in October 2017.
Since then, violence between armed anglophone rebels and government forces occurs almost daily in the restive provinces.
Abductions have been proliferating, with officials, foreigners and locals alike targeted.
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