The Egyptian prosecution is questioning prominent leftist and labour activist Kamal Khalil on charges of inciting illegal protest, spreading false news and insulting the president, Al-Ahram Arabic news website reported.
Khalil was arrested early on Thursday from his home.
Prosecutors interrogated Khalil over posts on his Facebook account and files on his personal laptop, according to his lawyer Khaled Ali.
The 67-year-old activist had participated in a sit-in last week at the press syndicate in Cairo to protest against the Egypt-Saudi Red Sea island deal.
The deal was approved by parliament in a majority vote on 14 June.
Small protests against the parliament decision were dispersed by police, with a number of people arrested. Many of those arrested were later released on bail.