In a statement, the Cairo Institute for Human Rights Studies said Amr Salah was seized outside his home on Thursday, citing an eyewitness saying he was beaten by four men in civilian clothes who put him into a vehicle with darkened windows.
The men identified themselves to the eyewitness as members of Egypt's security services, the statement said.
The institute's director, Bahey el-Din Hassan, later told Reuters that Salah had contacted his mother by telephone on Friday to say he would be released soon but had not given any details about who was holding him or his whereabouts.
"We have an illegal process, someone kidnapped from a street ... by men in civilian clothes and no information reported to his family (by the authorities)," Hassan said, adding he was seeking more information on how Salah was treated.
Interior Ministry officials could not immediately be reached for comment.
"The incident is especially worrying because other Egyptian activists have been forcibly disappeared in the past under similar circumstances," the group said in its statement, adding that Salah was a rights activist and researcher.
Two policemen went on trial in July charged with the illegal arrest and torture of anti-corruption activist Khaled Said who died in their custody, a case that has provoked protests at home and abroad.
That case has become a rallying point for Egyptians who say security forces act with impunity under an emergency law allowing indefinite detention and curbs on anti-government activity. The government says the emergency law is aimed at dealing with drug and terrorism cases.
