Human Rights Watch on Sunday condemned the deaths of more than 70 people in violence that erupted at protests in Egypt, accusing authorities of a "criminal disregard for people's lives".
At least 72 people were killed in Cairo alone on Saturday morning, according to the health ministry, at a rally of supporters of ousted Islamist president Mohamed Morsi.
Protesters accused security forces of opening fire with live ammunition, but the interior ministry said only tear gas had been used.
Another nine people were killed a day earlier during protests in Alexandria, the health ministry said.
New York-based HRW said many of those killed in Cairo were shot in the head or chest, and that medical staff interviewed by their researchers "judged some of the deaths to be targeted because of the position of the shots".
The deaths came after rival rallies by supporters and opponents of Morsi on Friday.
The protests followed a call by army chief General Abdel Fattah al-Sisi, who led the coup that ousted Morsi, for rallies to support a crackdown on "terrorism".
The deaths suggested "a shocking willingness by the police and by certain politicians to ratchet up violence against pro-Morsi protesters," HRW's deputy Middle East and North Africa director Nadim Houry said in a statement.
"It is almost impossible to imagine that so many killings would take place without an intention to kill, or at least a criminal disregard for people's lives."
The group urged Egypt's interim government and military leaders to "immediately order an end to the use of live gunfire except where strictly necessary to protect life."
Morsi, an Islamist from the Muslim Brotherhood, was ousted from the presidency on July 3 by the military after massive demonstrations against his rule.
The deaths in Cairo on Saturday were the bloodiest incident since his ouster, and raised new fears about the country's transition.