Over 300 people, including nine journalists, were estimated to be in custody in Egypt on Tuesday after security forces foiled attempted demonstrations the previous day, according to information from rights groups and the country's journalists' union.
Amnesty International condemned the crackdown on the planned protests against the handover of two strategic islands to Saudi Arabia, saying that authorities "appear to have orchestrated a heavy-handed and ruthlessly efficient campaign to squash this protest before it even began."
Large numbers of heavily armed security forces had blocked off protest sites from early on Monday, letting only flag-waving supporters of President Abdel-Fattah al-Sissi through their cordons.
Two small demonstrations in western Cairo were quickly dispersed by security forces using tear gas, according to local media reports.
The crackdown came after al-Sissi, the police and army all issued warnings against attempts to disturb security or break the country's laws, which effectively ban protests without prior police permission.
Dozens of people were also arrested ahead of the planned protests, with some being charged with incitement offences under a harsh new terrorism law, according to rights groups.
Authorities appeared to have been roiled by protests on the issue earlier in the month, which although limited in scale were among the largest non-Islamist demonstrations since al-Sissi took office in 2014.
The government sparked widespread anger when it quietly announced the planned handover of the islands of Tiran and Sanafir during a recent visit to Cairo by Saudi Arabia's King Salman.
Lawyer Ragio Omran, of the Front to Defend Egypt's Protesters, told dpa that the number in detention in Cairo alone was probably about 300, but the picture was not entirely clear yet.
The Egyptian Centre for Economic and Social Rights, an NGO, said 22 people had been arrested in the northern city of Alexandria.
State newspaper al-Ahram, quoting an unnamed security official, said that 285 people had been arrested in the capital. Once they were examined they would be released or sent to prosecutors, the official said.
Gamal Abdul-Rahim, secretary general of the Egyptian Journalists Syndicate, told dpa that 44 journalists had been arrested on Monday and nine were still in custody on Tuesday afternoon. Contacts were under way to have them released, he said.
The New York-based Committee to Protect Journalists (CPJ) condemned the arrests of journalists, adding that Egypt was "the second worst jailer of journalists worldwide in 2015."
CPJ figures showed Egypt held 23 journalists at the year's end, including photojournalist Mahmoud Abu Zaid who was arrested while covering the violent police dispersal of Islamist protests in August 2013.
He currently faces trial along with over 700 other defendants on charges that reportedly include illegal assembly and murder, according to CPJ and press reports.
Egyptian authorities have cracked down on dissent since al-Sissi, then head of the armed forces, ousted Islamist president Mohammed Morsi in July 2013.