Moroccans wanted to show support for the Palestinians
A crowd estimated to at least one million people has gathered in the centre of the Moroccan capital, Rabat, chanting slogans and waving Palestinian flags and banners.
Some American flags have been set alight and riot police line the streets, but there has been no violence so far.
The demonstration has been called by the Moroccan Association in Support of the Palestinian Struggle to denounce what it calls Israel's crimes against the Palestinians and to call for peace.
There have been unofficial protests and strikes by high-school students all over the country for the last two weeks, but this one is official.
It has the backing of all the main political parties, some of whom have been bussing in supporters from around the country.
With the demonstrations, the authorities may be hoping to channel the strong pro-Palestinian and anti-Israel feelings among ordinary Moroccans.
Morocco has traditionally played a role as peace-maker and intermediary in the Middle East.
And the Washington Post reports that US Secretary of State Colin Powell will stop off in Morocco for talks with King Mohammed VI before going on to talks with other Arab leaders and Israel.