An army spokesman told Reuters that Amnesty’s allegation, the latest in a series of charges levelled by the campaign group against Nigeria’s military in the last year, aimed to tarnish the security forces’ reputation.
The police said they did not attack people holding demonstrations.
Amnesty said the military fired live ammunition with little or no warning to disperse members of the Indigenous People of Biafra (IPOB) group between August 2015 and August the following year.
The unrest in the region is another challenge for the country’s president,Muhammadu Buhari, who is grappling with a sharp slowdown in Africa’s biggest economy, as well as the bloody Boko Haram insurgency in the north-east, and militancy elsewhere in the oil-rich southern Delta region.
Amnesty’s 60-page report based on interviews with 193 people, 87 videos and 122 photographs, said troops and the police used “arbitrary, abusive and excessive force to disrupt gatherings”.
Local media had previously reported “massive extra judicial killings” of pro-Biafra activists in Nigeria’s south-east.
Amnesty corroborated many of these claims, saying that at least 60 people were killed and 70 injured in two days in May after campaigners gathered for a rally in Onitsha , Anambra state.
Secessionist feeling has simmered since the separatist rebellion by the eastern Igbo people, one of Nigeria’s largest ethnic groups, led to a three-year civil war that ended 46 years ago. An estimated million people died, mostly from starvation and illness, in the conflict, which left deep scars and deep resentment.
Analysts say many of the same factors which prompted earlier anger still exist, and describe “a cocktail of longstanding and recent economic and political grievances”.
Now, like then, Igbos say they have been marginalised by being excluded from key government posts and denied vital funding for infrastructure development, schools and hospitals.
Anger in Biafra flared last year after the IPOB leader, Nnamdi Kanu, who was based in the UK, was detained on a visit to Nigeria and charged with criminal conspiracy and with belonging to an illegal society.
The arrest of Kanu, who set up an underground radio station in London, which authorities say called for violent attacks on Nigerian security forces, prompted supporters to hold protests. These were dispersed with live ammunition, according to Amnesty.
Don Awunah, a Nigeria police force spokesman, said that officers “always abide by the law” and adhere to best practices. “We don’t attack people who are demonstrating, which every Nigerian has a right to do,” he said.
Sani Usman, an army spokesman, said Biafra separatists had behaved violently, killing five policeman at a protest in May and attacking both military and police vehicles. “The military and other security agencies exercised maximum restraints despite the flurry of provocative and unjustifiable violence,” said Usman.
Witnesses told Amnesty that some protesters had thrown stones, burned tyres and, in one incident, shot at the police, but added that “these acts of violence did not justify the level of force used against the whole assembly”.
Makmid Kamara, interim director of Amnesty International Nigeria, said: “This reckless and trigger-happy approach to crowd control has caused at least 150 deaths.”
The campaign group said their research showed a disturbing pattern of hundreds of arbitrary arrests and ill-treatment by soldiers during and after IPOB events, including arrests of wounded victims in hospital, and torture and other ill treatment of detainees.
One interviewee said he was shot during a protest meeting and hid in a gutter. When soldiers found him they poured acid on him, he said.
Last year Amnesty said that more than 8,000 people had died in detention during a crackdown on Boko Haram. The group also said soldiers killed hundreds of Shia Muslims in the northern city of Zaria in December 2015. A judicial inquiry in August concluded that 347 people were killed and buried in mass graves after those clashes.
Nigeria has at least 170 million inhabitants, split roughly equally between Christians and Muslims across about 250 ethnic groups, who mostly co-exist peacefully.
Buhari, a former military dictator in the 1980s, was a brigade major who commanded troops in Biafra during the war in which soldiers were accused of mass atrocities. Last year Buhari said he would not let any secessionist campaign in Biafra succeed. “We will not let that happen. For Nigeria to divide now, it is better for all of us to jump into the sea and get drowned,” he said.
One grievance among some who support Biafran independence is that Nigeria’s presidents have tended to come from the north or south-west – areas dominated by Hausa and Yoruba people – which, they say, has led to Igbos not being appointed to influential government positions.
In 2012 a surge in protests led to the arrest on treason charges of more than a 100 supporters of a secessionist group after an independence rally in Enugu, the capital of Nigeria’s south-east region. The protesters included many elderly war veterans from the bloody 1967 conflict.
The arrests came shortly after the renowned Nigerian writer Chinua Achebe wrote in the Guardian that persecution of Igbos still persisted in Nigeria. Achebe’s memoirs prompted renewed debate about the 1960s conflict. Most of those involved in the current campaign for independence for Biafra are too young to remember the earlier war.
Analysts do not believe those campaigning for Biafra will make common cause with other militants in the Niger delta.
“Most groups in the delta are demanding regional autonomy and the right to control their petroleum resources within Nigeria. They are fiercely opposed to any suggestion of joining the Igbos in a breakaway Biafra,” wrote the International Crisis Group last year.