YAOUNDE, CAMEROON—Cameroon’s election commission says more than 20 candidates have registered to run against longtime President Paul Biya in October elections.

Opposition parties earlier this year discussed collectively backing Akere Muna, a prominent lawyer who specializes in fighting corruption, but now he is running only for his People’s Development Front party.

The decision to not form an opposition coalition likely will cement Biya’s win in the Oct. 7 vote.

The 85-year-old leader has been in power since 1982 and is Africa’s second-longest serving president. He oversees an increasingly restive Central African nation that faces not only Anglophone separatists but also the threat from Boko Haram extremists crossing the border from Nigeria.

If re-elected, Biya will rule until 2025.