At least 1,900 people have been detained since protests against President Abdel-Fattah al-Sissi began in Cairo and other Egyptian cities last Friday, the head of a rights monitoring group said on Thursday.
Gamal Eid, head of the Arab Network for Human Rights Information, said the arrests had been made between Friday and Wednesday and described them as part of the "most violent campaign in Egypt's (modern) history".
An Interior Ministry spokesman could not immediately be reached for comment.
>> Read more: The Egyptian army is making a fortune in Sinai | Analysis ■ Egypt praised for economic reforms, but millions of Egyptians barely survive | Analysis
Protesters chanting "Leave, Sissi" gathered in Cairo and other cities on Friday following online calls for demonstrations against alleged government corruption. The protests continued in the Red Sea city of Suez on Saturday.
There have been calls for similar protests this Friday, though supporters of the government are also planning mass rallies to show their support for Sissi.
Security forces have stepped up their presence in main squares in major cities and have been checking mobile phones for political content.
The protests have unnerved investors and led to a pro-Sissi campaign in Egypt's strictly controlled media.
On Thursday the front page of private pro-government newspaper Al Dostour carried pictures of 33 opposition figures, activists, journalists and writers under the headline: "Beware of them."
Among them was Mohamed Ali, a former contractor and actor who posted a series of videos in the run-up to the protests from his home in Spain, accusing Sissi and the military of corruption. Sissi has dismissed the allegations as "lies and slander".
Sissi, who came to power after leading the overthrow of Islamist former president Mohammed Morsi, has overseen a broad crackdown on dissent that has extended to liberal as well as Islamist groups, and which rights groups say is the most severe in living memory.
Sissi's backers say the crackdown was needed to stabilise Egypt after the turmoil that followed the country's 2011 uprising.
Want to enjoy 'Zen' reading - with no ads and just the article? Subscribe todaySubscribe now