The global human rights monitor said US and French military were also seen at one site where the abuses allegedly took place and urged Washington and Paris to investigate.
Boko Haram’s Islamist insurgency has devastated northeast Nigeria since it began in 2009, and the violence has spread to neighbouring countries, including northern Cameroon.
Cameroonian troops are part of a regional military force that has been successful in forcing them out of captured territory in northeast Nigeria since early 2015.
Amnesty’s regional director for West and Central Africa, Alioune Tine, said Boko Haram had committed atrocities and war crimes in Cameroon.
“But nothing could justify the callous and widespread practice of torture committed by the security forces against ordinary Cameroonians, who are often arrested without any evidence and forced to endure unimaginable pain,” he said.
“These horrific violations amount to war crimes,” he added in a statement, calling for an independent investigation.
– Crackdown –
Amnesty said it had unearthed 101 cases of secret detention and documented at least 24 different types of torture at more than 20 different sites in Cameroon between 2013 and 2017.
Four of the locations were military bases, two were run by the intelligence services, one was a private residence and another was a school.
But most of the torture happened at the headquarters of the Rapid Intervention Battalion (BIR), which has led the fight against Boko Haram, and the DGRE intelligence agency.
The majority of victims were men aged between 18 and 45 and from the Far North region, which has been repeatedly attacked by suicide and bomb attacks.
Women, children and people with physical and mental disabilities were also targeted. All were accused of support for the Islamic State group affiliate, it was alleged.
Torture methods included beating suspects with an electric cable while having water thrown on them, beatings to secure confessions and the use of excruciating stress positions.
Deprivation of food, water and medical treatment was also used; 32 of the 101 victims said they had seen people die because of the torture and ill-treatment.
The allegations — similar to those made against the militayr in Nigeria — are not the first against Cameroon, which has been criticised before for its heavy-handed crackdown.
Last year, more than 100 people were sentenced to death by a military court, while three students were jailed for 10 years for sharing a joke about Boko Haram by text message.
A Radio France Internationale reporter also received a 10-year prison term for allegedly supporting the jihadists.
– Abuse probe –
The report, “Cameroon’s Secret Torture Chambers: Human Rights Violations and War Crimes in the Fight Against Boko Haram”, said US and French troops were at one of the torture sites.
Washington in 2015 sent some 300 soldiers to northern Cameroon to conduct intelligence, surveillance and reconnaissance operations against Boko Haram.
That includes an unnamed drone operation over the restive border region with Nigeria, run from a base in the northern city of Garoua.
France, the former colonial power in Cameroon, runs its counter-terrorism operations against jihadist groups in the Sahel region from the Chadian capital, N’Djamena.
Amnesty said it had written to the US and French embassies in Cameroon to ask about their military’s presence at the BIR base in Salak, near the Far North region capital, Maroua.
Tine said Washington and other international partners “should investigate the degree to which their personnel were aware of illegal detention and torture at the Salak base”.
They should also establish whether troops reported it to their superiors and the Cameroon authorities, he added.
The US ambassador to Yaounde, Michael S. Hoza, wrote in response that it took seriously any allegation of human rights abuses by Cameroonian security forces.
Some BIR units in Salak and elsewhere were not receiving US assistance “because of credible information implicating those units in the commission of (gross violation of human rights)”, he added.
Amnesty said they had received no immediate response from the French embassy.