At least 2,000 protesters gathered in a square in Morocco's capital on Sunday to demand that King Mohammed give up some of his powers and clamp down on government corruption.
Some people in the crowd were waving Tunisian and Egyptian flags, a recognition of the popular uprisings there.
Uniformed police kept their distance from the protest, in Rabat's Bab El Ahad area, though there were plain-clothes officers mingling in the crowd with notebooks.
Analysts say Morocco, with a reformist monarch who is widely respected, and a growing economy, is one of the Arab countries least likely to succumb to the wave of protests sweeping the region.
Slogans chanted at the protest included: "The people reject a constitution made for slaves!" and "The people want the autocracy down!"
With heavy rain falling, people used plastic sheets as improvised raincoats.
"This is a peaceful protest to push for constitutional reform, restore dignity and end graft and the plundering of public funds," said Mustapha Muchtati of the Baraka (Enough) group, which helped organise the protest.
The protest was initiated by a group calling itself the February 20 Movement for Change, which has attracted 19,000 followers on the social networking website Facebook.
On the eve of the protest, a Moroccan youth movement said it was pulling out because of disagreements with Islamists and leftists.
Demonstrations were also planned for Morocco's other main cities, including Marrakesh, the top tourist destination.
Morocco is officially a constitutional monarchy with an elected parliament. But the constitution empowers the king to dissolve the legislature, impose a state of emergency and have a key say in government appointments including the prime minister.
Officials say Morocco's commitment to reform has never been as palpable as under King Mohammed who - as a member of the Alaouite dynasty that has been ruling Morocco for some 350 years and claims descent from the Prophet Mohammad - is considered sacred by the constitution.
The call for the protest has been portrayed as a healthy sign by the authorities. The government has worked since the king came to the throne in 1999 to repair a bleak legacy of human rights abuses, poverty and illiteracy left after the 38-year rule of his father, King Hassan II.
But Finance Minister Salaheddine Mezouar urged citizens to boycott the march, warning that any "slip may in the space of few weeks cost us what we have achieved over the last 10 years".
Officials have voiced concern that Algeria and the Polisario Front, which wants independence for the disputed territory of Western Sahara, may use upheavals sweeping some Arab countries to stir unrest. Morocco annexed Western Sahara in 1975.