Dozens of protesters belonging to the February 20 anti-government movement were attempting to hold a protest picnic in front of what they allege is a secret government detention centre in the Temara area where Islamists are held, a Reuters reporter at the scene said.
The government denies it runs secret detention centres. It later granted permission for the general prosecutor and national human rights council to inspect the headquarters of the domestic intelligence service, the state MAP news agency said.
Anti-riot police chased the protesters before the rally could begin, beating some with long rubber truncheons. Police said no arrests were made.
At a later rally on Rabat's outskirts, security forces beat into a coma one of the founders of the February 20 movement, Oussama ElKhlifi, a doctor and activists said. No independent verification of the report was immediately available.
The February 20 movement is linked to reform protests and uprisings this year elsewhere in the Arab world.
"This is the latest in a series of violent interventions by the security forces which shows that the authorities no longer tolerate the group's peaceful protest," said Nizar Benmate of the movement.
The government's chief spokesman Khalid Naciri was quoted as saying by the private Atlantic radio station that the Temara protest was broken up because it had been banned.
He denied there was a secret detention facility in the vicinity, saying the building singled out by the protesters was a local government administrative office.
Moroccan officials deny allegations from opposition groups and some human rights campaigners that they run secret detention centres and say all detainees are treated in strict accordance with the law.
Human rights activists say Islamists, political activists and others have been held and tortured at the Temara building.
MAP did not say when the inspection visit to Temara would take place. King Mohammed created the human rights council in March amid the protests.
The agency said about 100 "extremists" blocked a road in the northern city of Tangiers. It reported a similar incident in Fez where it said some protesters carried knives and al Qaeda flags. There was no independent confirmation of the reports.
Some 50,000 people gathered in the evening for a pro-reform rally in Casablanca, organisers said. Independent sources put the number at around 15,000.
The national press union criticised authorities for rough treatment of journalists at the Temara protest. Security forces beat two journalists, manhandled two others and confiscated some equipment, a Reuters witness said.