A violent confrontation ensued during which a reporter said he witnessed police beating a pregnant woman and some young girls. "It was unusually violent," he said.
"The attack on the demonstrators began around 2:00 pm (1400 GMT)," Abderrahim Tafnout, a PSU leader, told AFP.
"The police could not get into the headquarters, particularly because of the resistance of the demonstrators who were beaten up."
Witnesses and reporters saw dozens of people who had been injured, some seriously.
"About 30 of the injured, 10 of whom were badly hurt, were taken to hospital," said Tafnout, who works as a journalist for state television. "Most of them had received blows to the head, while others had more minor wounds."
The assault was ordered by the governor of Casablanca. Beforehand, police had used megaphones to call on the demonstrators to leave the PSU headquarters and stop shouting slogans.
A journalist who did not want to be named said that "most of the demonstrators were activists of the Justice and Charity Party (PJB)," a movement whch is banned but tolerated by the authorities and is among the north African country's largest.
About 100 demonstrators, most of them PJB members, had been dispersed in the morning by security forces who prevented them holding a rally to demand political reforms.
A source in the security services told AFP that most of the demonstrators were Islamists who had sought refuge in front of the PSU building and had gathered without previous authorisation.
He said that about a dozen people had been injured, but in most cases only slightly. He said that no one's life was in danger.
"We were meeting in the political office and were about to publish a communique praising the king's speech last Wednesday when the security forces tried to break in," Mohamed Bouaziz, a PSU leader, told AFP.
On Wednesday King Mohammed VI announced major democratic reforms, strengthening the powers of the prime minister and broadening individual freedoms.
The announcement came in his first address to the nation since demonstrations on February 20 calling for more democracy and less corruption, part of a tide of contestation sweeping the Arab world.
The Moroccan media hailed the reforms as "historic" while raising questions about the future of some of the king's entourage.
The planned reforms were welcomed by the United States and the European Union.
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