Since Biya, the world's oldest head of state, was proclaimed the victor of the October 12 vote, the central African country has been shaken by demonstrations, whose repression saw the deaths of several protesters.

Opposition candidate Issa Tchiroma Bakary, a former Biya ally who has since turned against the veteran leader, insists that he is the rightful winner and has urged his supporters to continue rallying and engage in civil disobedience.

In a statement on Tuesday, HRW said that while some protests "were violent" and saw incidents of crowds assaulting the police, the Cameroonian authorities responded to the demonstrations through the use of "lethal force and mass arrests of protesters and other citizens".

At least six of those arrested are being held at the State Defence Secretariat in the capital Yaounde, a detention facility where the rights watchdog "has documented routine use of torture".

"The violent crackdown on protesters and ordinary citizens across Cameroon lays bare a deepening pattern of repression that casts a dark cloud over the election," said Ilaria Allegrozzi, a senior HRW Africa researcher.

"The authorities should immediately rein in, investigate, and prosecute responsible security forces, and all political leaders should call on their supporters to reject violence."

"The Cameroonian authorities should immediately release all those held in connection with peaceful protests or for peaceful expression of opposition to the government," Allegrozzi added, urging the authorities to give protesters involved in violence a fair trial.