Egypt's prosecution ordered a 15-day renewal of the detention of rights lawyers Malek Adly, Haitham Mohamadein, Sayed El-Banna and political activist Zizo Abdo pending further investigations.
Labour lawyer Mohamadein and activist Abdo are being questioned on charges of belonging to the outlawed Muslim Brotherhood, attempting to overthrow the regime and calling for protests against the redrawing of the country's maritime borders in a recent Egyptian-Saudi agreement, the Freedom for the Brave activist group said on its Facebook page.
The next court session is set for 1 June.
In a separate case, rights lawyers Adly and El-Banna face accusations of inciting to violate the protest law, attempting to destabilise the country and plotting to overthrow the regime.
Mohamadein and El-Banna were arrested from their homes on 22 April, while Abdo and Adly were arrested in Cairo's Maadi district on 6 May.
Dozens of activists were arrested on 22 April ahead of protests planned for 25 April, Sinai Liberation Day, against an Egyptian-Saudi deal to acknowledge Saudi Arabian sovereignty over the Red Sea islands of Tiran and Sanafir.