Over 2,000 people took to the streets of the capital Rabat, 4,000 according to the organisers, shouting: "The people want change."
In Casablanca, the North African nation's biggest city, over 1,000 people came out demanding: "Freedom, dignity and justice,".
The demonstrations were peaceful as of midday.
"I want a Morocco that's more fair and with less corruption," said a student demonstrator in Casablanca who asked not to be named.
"We've got nothing against the king, but we want more justice and work," said another student who gave his name as Brahim.
Thousands of young Moroccans have joined the "February 20" movement on the social networking site Facebook, calling for peaceful demonstrations demanding a new constitution limiting the king's powers and more social justice.
The call has similar origins to the so-called "Facebook revolutions" that toppled decades-old regimes in Tunisia and Egypt and sparked deadly protests in Bahrain, Yemen, Algeria and Morocco.
Ahead of the protest, Morocco promised to inject 1.4 billion euros in subsidies to soften price hikes for staples -- a key factor among others including rampant unemployment behind the spreading unrest in the Arab world.
That came despite an earlier reassurance that Morocco was unlikely to see Tunisia or Egypt-style unrest due to ongoing reforms by King Mohammed VI who has ruled the country for over a decade.
Human rights and civil groups as well as independent journalists joined the movement, calling for the adoption of a democratic constitution.
The youth wing of banned Muslim group Justice and Charity, believed to be Morocco's biggest opposition force called for a peaceful rally.
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