douala cameroon military court cameroon sentence opposition leader monday 25 years prison lawyer amnesty international say denounce trial politically motivate court convict aboubakar siddiki president northern cameroon main opposition party hostility homeland well revolution contempt president accusations plot destabilise country go appeal decision seem us siddiki lawyer emmanuel simh tell reuters statement amnesty say prosecution part government campaign stifle critics government deny charge political dozens people kill hundreds arrest crackdown recent months protest cameroon english-speaking regions residents say suffer social economic marginalisation predominantly francophone country protest become lightning rod opposition president paul biya 35-year rule besides siddiki court sentence abdoulaye harissou well-known notary three years prison failure denounce crime court also drop charge three journalists arrest connection case cameroon court sentence opposition leader 25 years prison