Egyptian undercover policemen detain protesters during a demonstration organized by the group "No Military Trials for Civilians" in front of the Shura council in downtown Cairo on Tuesday. (Khaled Desouki/AFP/Getty Images)

Egyptian police violently disbanded a small protest mounted by pro-democracy activists in central Cairo on Tuesday night, arresting dozens of well-known rights advocates just two days after the military-appointed interim president signed an acutely restrictive law regulating demonstrations in the turbulent country.

Security forces wielding fire hoses and tear-gas launchers charged a group of more than 100 demonstrators protesting a constitutional article granting the army authority to try civilians in military courts, chasing them onto side streets and detaining as many as 70, rights groups said.

The dispersal kicked off clashes between protesters and police in the streets around Cairo’s Tahrir Square, with police firing tear gas and protesters answering with rocks and burning tires. It marked the most direct assault by security forces on the Tahrir Square revolutionary activists — who oppose both the military and the Muslim Brotherhood — since an army coup ousted Islamist President Mohamed Morsi in July.

At least 13 of the 50 representatives on the committee writing Egypt’s new constitution said they were freezing their memberships until the detainees, including Mona Seif, a high-profile activist and co-founder of the No Military Trials for Civilians rights group, were released, state media reported.

Witnesses said police hauled some of the activists away in a van, but it was unclear where they would be held or what charges they would face. A leader of the Strong Egypt Party’s student movement, Mahmoud Omar, said his friend and prominent activist Ahmed Harara — who lost sight in both eyes in clashes with police in 2011 — is missing after the police assault.

Am anti-government activist runs with a flare in Cairo on Nov. 26, 2013. (Ahmed Gomaa/AP)

Fellow activists said some of those arrested were taken to a prison in the Cairo suburb of Tagammo. State news media said police had arrested 20 people; rights activists said as many as 74 were detained, including journalists.

In a statement on its official Facebook page, the Interior Ministry said that the protesters had “disrupted traffic” and thrown rocks at police, prompting security forces to respond with water hoses. The cabinet, meanwhile, said on Facebook that the prime minister had met with representatives of different political forces and that a committee would be formed to review the protest law.

Police acted under a law enacted this week by the military-backed government that severely curtails the rights of citizens to peacefully assemble and grants security forces broad authority to quash demonstrations, including with riot-control weapons and live ammunition.

The United Nations’ human rights chief, Navi Pillay, said Tuesday that the law must be amended or repealed, saying it endangers the lives of peaceful protesters.

Jen Psaki, a spokeswoman for the U.S. State Department, said Monday that the new measure, which can send violators to prison for up to three years, “does not meet international standards and will not move Egypt’s democratic transition forward.”

Egypt has seen near-daily demonstrations, both big and small, since a popular uprising ousted strongman Hosni Mubarak in 2011. Democracy activists mobilized protests against the military council that ruled from 2011 to 2012, and mass anti-Morsi rallies led the military to remove the unpopular leader in July.

Supporters of Morsi also have marshaled consistent street protests, despite a crackdown on his Muslim Brotherhood group that crippled the movement’s ability to organize. Anti-coup demonstrations at university campuses nationwide also are putting pressure on the interim government.

Sharaf al-Hourani contributed to this report.