Amnesty International called on Tuesday for an independent investigation into killings by Egyptian security forces as well as torture and violations of the rights to free speech and assembly.
The military's overthrow of President Mohamed Morsi in July after mass protests against him unleashed an "extreme level of political violence", the London-based group told the United Nations Human Rights Council.
"Between 14 and 18 August, at least 1,089 people were killed, many due to the use of excessive, grossly disproportionate and unwarranted lethal force by security forces," said Peter Splinter, Amnesty representative in Geneva.
Egyptian security forces have also failed to prevent or end a wave of sectarian attacks targeting Coptic Christians, he said, referring to attacks on the minority who make up 10 percent of its 85 million people.
"The scale of human rights violations, including of the right to life, the right to fair trial, the right to be free from torture, the right to freedom of expression and assembly, demands an urgent, impartial, independent and full investigation," Splinter said, adding that the results of the inquiry should be made public.
On Monday, U.N. High Commissioner for Human Rights Navi Pillay reiterated her call for an independent inquiry into the killings, as well as her request to send a team to Egypt to assess the situation.
"The path to stability in Egypt lies in its ability to establish the rule of law in an inclusive manner that ensures that all Egyptians, irrespective of their political opinion, gender, religion, or status, are recognized as legitimate stakeholders in the future of their country," she said.