Former Egyptian presidential candidate and rights lawyer who challenged the government for handing over islands to Saudi Arabia was detained Tuesday on suspicion of illegal political work, a judicial official said.
Khaled Ali was summoned by the prosecution for questioning on suspicion of having set up an unregistered political party, the official said.
The prosecution ordered his detention for 24 hours.
His legal troubles come as opposition groups complain of a crackdown ahead of the 2018 presidential election which incumbent Abdel Fattah al-Sisi is expected to dominate.
Ali was the main lawyer to bring a case against the government after it agreed to hand over two Red Sea islands to Saudi Arabia in April last year.
The agreement to transfer the islands of Tiran and Sanafir sparked rare protests in Egypt, which bans all but court approved demonstrations.
Ali also ran in the 2012 election that brought to power Islamist president Mohamed Morsi.
Dissidents accuse Sisi of having installed a repressive regime since the then army chief ousted the unpopular Morsi in 2013.
But Sisi also enjoys widespread support among Egyptians, who say the country needs a firm hand to cope with years of tumult and a deadly jihadist insurgency.