Amnesty International on Monday condemned the police's use of deadly force against demonstrators in Egypt, where nearly 50 have been killed in clashes.
"Eyewitness accounts collected by Amnesty International in Egypt point to the unnecessary use of lethal force by security forces during a weekend of clashes with demonstrators," it said.
It said an Amnesty researcher probing killings in Suez had collected "disturbing eyewitness accounts of excessive force, including in some instances security forces using lethal force when it was not strictly necessary to protect life."
Amnesty's deputy Middle East and North Africa head, Hassiba Hadj Sahraoui, said: "Egyptian authorities must issue clear orders to those policing protests to respect freedom of peaceful assembly and avoid unnecessary or excessive force.
"They must make absolutely clear that those that use arbitrary and excessive force will be brought to justice... The recourse to violence by some protesters does not give a blank cheque to the police to shoot and beat protesters."
Violence has swept Egypt since Thursday evening, prompting authorities to declare a month-long state of emergency in the provinces of Port Said, Suez and Ismailiya, where rioters have attacked and torched police stations.
In Cairo on Monday, one man was killed as the clashes between police and protesters raged into a fifth day.
The bloodshed has focused on Port Said where riots were sparked sparked by death sentences handed down on Saturday against supporters of local football club Al-Masry over stadium violence last year that killed 74 people.
On Sunday, angry anti-riot policemen barred Interior Minister Mohammed Ibrahim from attending the funeral in Cairo of two colleagues killed in Port Said, the official news agency MENA reported.
It said the action was taken in protest at police not having been armed with live rounds to protect themselves.