Amnesty accused the army of using weapons suitable for the battlefield to kill protesters. It said they were in the hands of units accused by rights groups of years of atrocities against minority ethnic groups, including Rohingya Muslims.
"These are not the actions of overwhelmed, individual officers making poor decisions," said Joanne Mariner, director of Crisis Response at Amnesty International.
"These are unrepentant commanders already implicated in crimes against humanity, deploying their troops and murderous methods in the open."
Amnesty said weapons used included sniper rifles and light machine guns, as well as assault rifles and sub-machine guns.
It called for a stop to the killings and for the release of detainees. The Assistance Association for Political Prisoners says nearly 2,000 people have been detained since the coup.
In justifying its takeover, the army cited alleged fraud in a November election that Suu Kyi's party had won. Its accusations had been dismissed by the electoral commission.