A Commission of Inquiry on human rights in Eritrea found that crimes against humanity have been committed in the country since 1991.
Those included the persecution of Christians, forced labour, rape, murder and unlawful detention, the report said.
Chair Mike Smith confirmed it has concluded that "crimes against humanity have been committed in a widespread and systematic manner in Eritrean detention facilities, military training camps and other locations across the country over the past 25 years".
Perpetrators were identified as high-ranking members of the government, ruling party and the military.
The report also concludes there are "reasonable grounds to believe that the National Security Office is responsible for most cases of arbitrary arrest, enforced disappearance and torture in official and unofficial detention centres."
Eritrea dismissed the report as "legally indefensible", adding that "Eritrea rejects the politically motivated and groundless accusations and destructive accusations of the commission."
Christian Solidarity Worldwide's Chief Executive Mervyn Thomas said: "CSW applauds the commitment, integrity and dedication of the COIE, which worked meticulously under difficult conditions to give a voice to Eritrean victims.
"The Commission's findings regarding crimes against humanity will come as no surprise to most long term observers of events in in Eritrea.
"It is deeply disheartening that grave and egregious violations have been occurring for 25 years in a country that waged a heroic independence struggle ostensibly to secure justice and human rights for its citizens, yet the people are brutalized and the nation still awaits the dividends of this hard-won freedom.
"Justice delayed is justice denied, and twenty-five years is too long. We therefore urge international bodies and UN member states to support the findings of this report and ensure its recommendations are acted upon in a timely manner.
"The international community must move swiftly to end the prevailing impunity by identifying and holding all those found to be responsible for crimes against humanity to account."