Dozens of people in Egypt have apparently disappeared after being detained secretly by security forces, Human Rights Watch (HRW) has warned.
The US-based group said in some cases officials denied holding individuals or refused to reveal their fates.
It called on the authorities to immediately disclose their whereabouts and hold those responsible to account.
An Egyptian official rejected the report as "lies" and part of a campaign to embarrass Egypt.
Speaking to the state-run Mena news agency, the unnamed official said: "The report was based on undocumented sources that did not provide true information or data on the issues it tackled."
The official went on to say that most of those named in the report had been "convicted in criminal cases or held pending trials".
Enforced disappearances constitute a serious violation of international law.
If carried out systematically as a matter of policy, they are a crime against humanity.
HRW's deputy Middle East director Joe Stork said that under President Abdul Fattah al-Sisi, who as military chief led the overthrow of President Mohammed Morsi in 2013, Egypt's security forces operated with "nearly absolute impunity".
Hundreds of people have been killed and more than 40,000 believed to have been detained, in a crackdown on dissent, HRW says.
Most were members or supporters of the outlawed Muslim Brotherhood but also secular and liberal activists.
HRW documented the cases of five people "forcibly disappeared" between April 2014 and June 2015, and two most likely to have suffered the same fate.
In three cases, the victims were last seen in the custody of state officials, although the authorities initially denied that the people had been detained or refused to reveal their whereabouts.
In three others, relatives and others who knew the disappeared said security forces had detained them. A doctor who disappeared remains unaccounted for.
The whereabouts of three of the individuals were determined only days or weeks after their disappearances either because state authorities eventually acknowledged their detention or because other people saw them in official custody.
Three university students, Esraa al-Taweel and her two friends Omar Ali and Souhaib Saad, all in their early 20s, were apparently arrested on 1 June as they walked along the River Nile in Cairo's Maadi district, according to their families.
Interior ministry officials repeatedly denied arresting any of them but more than two weeks later relatives saw them in various detention facilities, HRW said.
Three other victims of enforced disappearances were found dead after a period during which their whereabouts were unknown, HRW said.
Al-Sayed al-Rassed, 46, was arrested on 4 June in Banha, the capital of Qalyubia province, by several police in civilian clothes and a Central Security Forces officer and soldiers in uniform, his son Mohamed said.
They did not say where they were taking him. Three days later, his family received a call from the mayor's office informing them that his body was at a mortuary.
HRW said Egyptian human rights organisations had credibly documented scores of additional cases of enforced disappearances in 2015 and in some from 2013.
In June, Freedom for the Brave, an independent group offering support to detainees, documented what it said were 164 cases of enforced disappearance since April and said that the whereabouts of at least 66 remained unknown.
It listed 64 people whose whereabouts were revealed after more than 24 hours, the maximum time allowed to detain someone without charge under Egyptian law.