The family of prominent Palestinian politician Nabil Shaath accused Egyptian authorities on Wednesday of arresting Shaath's son Ramy last month.
"Ramy was arrested on Friday July 5th at 12:45 AM, from his home in Cairo," the family said in a statement on Facebook.
"At least a dozen heavily armed security agents stormed and searched his residence without presenting any legal document," they added.
The son of a former interim prime minister and Oslo peace process negotiator, Ramy also entered politics and served as an advisor to Palestinian Liberation Organization head Yasser Arafat.
The 48-year-old has lived since 1977 in Egypt, where he has citizenship.
His arrest came 10 days after authorities raided 19 businesses allegedly tied to the banned Muslim Brotherhood, accusing them of funding a plot to overthrow the state.
Among those also arrested were several prominent secular activists including former lawmaker Zyad el-Elaimy.
After disappearing for 36 hours, Shaath appeared before prosecutors and was charged with aiding a "terrorist group" connected to the same plot, the family said.
"His arrest is a disaster," said his wife Celine Lebrun-Shaath, a French national, who was deported from Cairo shortly after his arrest.
Speaking to AFP from Paris, where she is working for his release, she said negotiations with Egyptian authorities had not yielded any results, prompting the family to publicise his case.
Shaath has been detained in Tora prison, one of Egypt's most notorious jails, for around six weeks.
His family said they have been able to visit him regularly but are concerned about his health, as he suffers from high cholesterol.
Shaath is Egypt coordinator of the Boycott Divestment and Sanctions (BDS) movement against Israel.
He has been critical of Egypt's participation in Washington's controversial Israeli-Palestinian peace plan.
Most recently, he criticised Cairo for sending a mid-level official to a June conference in Bahrain where US President Donald Trump's son-in-law Jared Kushner launched the initiative.
His wife Celine said she felt "powerless... not knowing when I will see him again".
"We want (Egypt) to release Ramy. He has not done anything that he is accused of," she said.
"He is being arrested for his peaceful activities."