Hundreds of Egyptians took to the streets on Friday and Saturday in rare demonstrations calling for the departure of President Abdel Fattah al-Sisi.
"The latest count of those arrested over the events of since Friday in Cairo and other provinces has risen to 516," the Egyptian Centre for Economic and Social Rights said in a statement.
On Sunday, Egyptian authorities also detained an award-winning human rights lawyer after she attended judicial investigations into the arrests of protesters.
A judicial official said Mahienour el-Massry had been placed in pre-trial detention for 15 days on charges of "taking part with a terrorist group to achieve its goals and publishing false news".
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Massry, a political activist and human rights defender, has been tried and jailed twice for taking part herself in demonstrations.
Egypt effectively banned protests under a law passed following president Mohamed Morsi's 2013 ouster by the army.
Sisi was elected president the following year with 96.9 per cent of the vote.
But discontent over rising prices has been swelling in Egypt, where the government has imposed strict austerity measures since 2016 as part of a $12-billion loan package from the International Monetary Fund.
On Saturday, protesters headed to downtown Suez for the second night in a row, where they were met by security forces barricading the streets and armoured vehicles.
The protests came after Mohamed Aly, an exiled businessman and opponent of Sisi posted calls online for demonstrations against Sisi.
He has called a "million-man march" next Friday and for protesters to fill all "major squares" of the country.
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