Egyptian authorities issued stern warnings on Sunday, a day ahead of planned protests against the handover of two strategic Red Sea islands to Saudi Arabia.
President Abdel-Fattah al-Sissi, in a televised speech, said that "evil forces" were attempting to threaten the country's institutions.
"It is our responsibility, all of us, to preserve this security and stability," al-Sissi said.
April 15 saw the largest non-Islamist protests against al-Sissi's government in two years, amid public anger over the plan to hand over the islands of Tiran and Sanafir to Saudi Arabia.
Activists have called for further protests on Monday, a national holiday marking the return of the Sinai Peninsula - including the two islands - to Egyptian control under a peace treaty with Israel.
The government's conclusion that the islands were originally Saudi, and its agreement to hand them over, were quietly announced during a recent visit to Cairo by Saudi King Salman, provoking outrage on social media and among the Egyptian opposition.
Egyptian Interior Minister Magdi Abdel-Ghaffar, who heads the country's police force, said that there would be no leniency towards anyone breaking the law.
Protests without prior police approval have been effectively banned under a law passed months after al-Sissi, then head of the armed forces, ousted Islamist president Mohammed Morsi in mid-2013.
"The law will be implemented upon everyone with absolute firmness and nobody will be allowed to contravene it under any pretext," Abdel-Ghaffar told police commanders.
Dozens of people, including activists suspected of involvement in the second day of protests called for Monday, have been arrested in recent days, according to rights lawyers.
Prosecutors have accused a number of them of terrorist offences such as inciting attacks on police stations and the overthrow of the regime, lawyer Mokhtar Mounir of the Association for the Freedom of Thought and Expression told dpa.