Protesters accused security forces of heavy-handedness when human rights activists and intellectuals gathered outside Parliament on Wednesday.
According to demonstrators, police moved in and violently broke up the gathering using batons.
Security forces said the protest had not been officially authorised and had to be peacefully dispersed. But a spokesman confirmed scuffles did break out among protesters, some of whom had fallen to the ground.
Among those caught up in the incident was Abd al-Hamid Amin, head of the Moroccan Association for Human Rights .
"Policemen hit him on the back and on the head," said Abd al-Ilah Bin Abd Islam, who was with Amin at the time. "They hit him to please the Americans," he added while helping Amine to get back on his feet.
Other witnesses said film-maker Nabil Ayush was also struck.
Not so free trade
The protest, called by a trade union, was against the planned US-Morocco free trade agreement, with those taking part denouncing clauses which they said threatened cultural diversity and access to health care in the kingdom.
Moroccan farmers have voiced fears a flood of cheap American imports can undermine the agricultural sector which employs about half of the population.
Pharmaceutical firms and health groups have said the deal will drive up drug costs for the majority of Moroccans.
Cultural groups are concerned about the protection of Moroccan traditions.
The accord, still being negotiated, is expected to be signed in April or May.
Morocco has already embarked on a 12-year gradual free trade accord with the European Union that will reach maturity in 2012.