(CNN) Nigerian security forces have killed at least 150 people in a series of attacks against the largely peaceful pro-Biafra secessionist movement, according to a report by Amnesty International.
The report finds "overwhelming evidence that Nigerian security forces committed gross human rights violations" at a number of public gatherings in the country's southern region since August 2015.
In what the human rights group calls "a chilling campaign," security forces were apparently responsible for the extrajudicial killings, detentions, and torture of supporters of the Indigenous People of Biafra (IPOB), a group that is pushing for an independent Biafran state in Nigeria's oil-rich southeast.
"This deadly repression of pro-Biafra activists is further stoking tensions in the southeast of Nigeria," said Makmid Kamara, Interim Director of Amnesty International Nigeria.
"This reckless and trigger-happy approach to crowd control has caused at least 150 deaths and we fear the actual total might be far higher," Kamara said.
The report was based on the analysis of 87 videos and 122 photographs, along with 146 witness interviews.
It states that many of the abuses occurred around the Biafra Remembrance Day celebrations on May 30, 2016, in Onitsha, Anambra State. Security forces raided homes the night before the event and then opened fire the next day with live ammunition on a crowd of approximately 1,000 people, the report says.
A 26-year-old man interviewed in the report said was at the rally when he was shot and then hid from security forces in a gutter.
He said that when soldiers found him they poured acid on him.
"I covered my face. I would have been blind by now," he told Amnesty International. "He poured acid on my hands. My hands and body started burning. The flesh was burning... They dragged me out of the gutter. They said I'll die slowly."
The report acknowledges that some protesters threw stones, burned tires and in at least one incident, shot at police.
"These acts of violence and disorder did not justify the level of force used against the whole assembly," the report reads.
Nigerian military spokesman Colonel Sani Usman said: "We wish to debunk the insinuation that our troops perpetrated the killing of defenseless agitators.
This is an outright attempt to tarnish the reputation of the security forces in general and the Nigerian Army in particular, for whatever inexplicable parochial reasons.
"For umpteenth times, the Nigerian Army has informed the public about the heinous intent of this Non-Governmental Organisation," Usman added, "which is never relenting in dabbling into our national security in manners that obliterate objectivity, fairness and simple logic."
The Biafra movement calls for an independent state in the majority Igbo region in Nigeria's oil producing southeast. Supporters of the independence movement claim the region has been marginalized from government participation and development.
The Igbo are one of the largest ethnic groups in Africa's most populous nation, and their historical grievances carry over after the Nigerian Civil War, which started in 1967 and was largely fought to repress the Igbo secessionist movement. The Igbo lost the war, which ended in 1970.
For more than a year the IPOB have been staging a series of rallies seeking the establishment of a Biafran state.
Protests intensified in October 2015 when Nnamdi Kanu, leader of the IPOB, was arrested. He remains in detention.
The rights group calls for reparation for the victims as well as impartial investigations into those who perpetrated the abuses, which it says have not been adequately addressed by Nigerian authorities.
"Amnesty International has repeatedly called on the government of Nigeria to initiate independent investigations into evidence of crimes under international law, and President (Muhammadu) Buhari has repeatedly promised that Amnesty International's reports would be looked into," said Amnesty's Kamara.
However, "no concrete steps have been taken," he said.