Egyptian prosecutors questioned a number of protesters on Saturday who were arrested a day earlier while demonstrating against a government decision to hand over the Red Sea islands of Tiran and Sanafir to Saudi Arabia.
Several thousand people demonstrated in downtown Cairo on Friday in one of the biggest protests in recent months.
Dozens of protesters were arrested at the rally and at demonstrations in other governorates, security sources had said, after teargas was fired to disperse demonstrators. Many of them were later released.
Prosecutors in central Cairo opened an investigation on Saturday into the 26 protesters who were arrested at the main demonstration outside the Journalists Syndicate in downtown Cairo, a judicial source told Ahram Online.
They face charges of "protesting without a licence" and "possession of fireworks."
A controversial 2013 law bans all but police-sanctioned demonstrations; violators are often jailed.
The interior ministry has not provided the total number of those arrested during Friday's protests.
During a visit by Saudi's King Salman last week, the Egyptian government announced it was handing over the two islands in the Gulf of Aqaba to Saudi Arabia.
The announcement has sparked widespread public outcry, with critics accusing President Abdel-Fattah El-Sisi of "selling" the islands.
Egyptian and Saudi officials say the islands belong to the Gulf kingdom and were only under Egyptian control because Riyadh had asked Cairo in 1950 to protect them.