Cairo: Thousands of Egyptians angered by President Abdel Fattah Al Sisi's decision to hand over two islands to Saudi Arabia called on Friday for the government to fall, chanting a slogan from the 2011 protests.
Egyptian security forces detained about 50 protesters, according to two security officials. Police surrounded crowds at the press syndicate, site of the biggest demonstration.
Sisi's government prompted an outcry in Egyptian newspapers and on social media last week when it announced a maritime demarcation accord that put the uninhabited Red Sea islands of Tiran and Sanafir in Saudi waters.
"The people want the downfall of the regime!" protesters cried outside the Cairo press syndicate, using the signature chant of the 2011 revolt against then president Hosni Mubarak, who later stepped down.
In other parts of Cairo, police fired tear gas at protesters, security sources said.
Saudi and Egyptian officials say the islands belong to the kingdom across the Red Sea and were only under Egyptian control because Riyadh had asked Cairo in 1950 to protect them.
Saudi Arabia and other Gulf Arab states showered Egypt with billions of dollars in aid and grants after Sisi toppled freely elected president Mohamed Morsi of the Muslim Brotherhood in 2013 after mass protests against him.
But a sharp drop in oil prices and differences with Cairo over regional issues such as the war in Yemen have raised questions over whether strong support is sustainable.
Egyptians are eager for an economic revival after years of political upheaval, but the islands issue seems to have hurt their national pride, and prompted thousands to return to the streets to challenge their leader.
Critics say the government has mishandled a series of crises from an investigation into the killing of Italian graduate student Giulio Regeni, 28, in Cairo to a bomb that brought down a Russian airliner in the Sinai Peninsula last October.
Torture marks on Regini's body prompted human rights groups to conclude he died at the hands of security forces, which Egypt denies.
Sisi has made fighting corruption a top priority. But he drew fire last month after sacking Hesham Geneina, Egypt's top auditor, who had stirred controversy by publicly concluding that state corruption had cost the country billions of dollars.
In a tweet, Geneina described the protests as the "purest, bravest and most noble demonstration of Egyptians" in decades.
Many Egyptians, eager for an end to the turmoil triggered by the 2011 protests, enthusiastically welcomed Sisi when he took over. They turned a blind eye as hardliners and other opponents were rounded up.
But a growing number are now losing patience over corruption, poverty and unemployment, the same issues that led to Mubarak's downfall.
"We want the downfall of the regime," said Abdelrahman Abdellatif, 29, an air conditioning engineer, at the Cairo protest. "The youth of the revolution are still here... We are experiencing unprecedented fascism and dictatorship."
There were also Sisi supporters, including a woman wearing a shirt with an image of the former military intelligence chief.
In Alexandria, around 500 people gathered near a railway station. Meanwhile, 300 Sisi supporters holding up photographs of him protested outside a mosque in the port city.
Calls for protests have gathered thousands of supporters on Facebook, including from the outlawed Brotherhood, which accused Sisi of staging a coup when it was ousted and rolling back freedoms won after hundreds of thousands of Egyptians protested five years ago in Cairo's Tahrir Square against Mubarak.