Amnesty International claims the Nigerian military has fired live ammunition, with little or no warning, to disperse members of the Indigenous People of Biafra group since August last year.
Amnesty's report, based on interviews with almost 200 people, scores of videos and more than a hundred photos, says troops and police used excessive force to disrupt gatherings.
This man claims to have been a witness to one of the attacks.
"They started shooting us, killing our brothers. Over 10 of them were lying dead there."
Police argue they did not attack people holding demonstrations.
And the army says Amnesty's statement aims to tarnish the security forces' reputation.
Army spokesman Sani Usman says Biafra separatists have behaved violently, killing five police officers at a protest in May and attacking both military and police vehicles.
But witnesses told Amnesty, while some protesters had thrown stones, burned tyres and, in one incident, shot at police, the acts did not justify the level of force used against the group.
This woman says boys as young as 15 were among the victims.
"People were dying. And people were falling on the ground, blood gushing out. The whole part of that field was filled with human beings' blood. And after a while, they start carrying dead bodies into their van. Where they took them to is what we don't know."
Secessionist feeling has simmered in the country's south-east since the Biafra separatist rebellion in 1967 that pushed for an independent state.
It was flagged to be a homeland for the Igbo people, one of the country's largest ethnic groups.
The war, which raged for three years, is estimated to have killed a million people.
Tensions flared again after Indigenous People of Biafra leader Nnamdi Kanu was detained in October last year.
He was charged with criminal conspiracy and belonging to an illegal society.
Supporters held protests in response, and Amnesty claims they were dispersed with live ammunition.
The interim director of Amnesty International Nigeria, Makmid Kamara, has called on authorities to launch an investigation into the matter.
He says the increased militarisation in policing is disturbing.
"We are concerned about the increasing militarisation of routine policing functions, because we think it is creating a lot of scope for the Nigerian army to be killing -- what we would consider killing at will -- innocent people, people who hold a dissenting voice."
Mr Kamara says Amnesty is particularly concerned that, out of the 36 states in Nigeria, the military is currently deployed in 30.
"The increasing military presence in these states across Nigeria is, in itself, something that should raise concern for ordinary Nigerians and for everybody involved in ensuring that human rights are respected, protected and fulfilled. And we think that the current level of the use of military as the first point of call to address peaceful processions, peaceful gatherings and demonstrations is something that the Nigerian government has to stop, because, one, it gives the army what they consider a carte blanche excuse for killing innocent people, because the army is trained to kill and not to police processions or protests or demonstrations. They are trained to eliminate whoever they see as the enemy."
The report is the latest in a string of accusations Amnesty has levelled at the army.
It says more than 8,000 people died in detention during a crackdown on the militant group Boko Haram in 2015.
The rights group also says soldiers killed hundreds of Shiite Muslims in the northern city of Zaria late last year.
A judicial inquiry in August concluded 347 people were killed and buried in mass graves after those clashes.
