Egyptian riot police used water canon and batons to beat back crowds throwing rocks and trying to advance on the US embassy in Cairo.
"We just wanted to tell the Americans they are no longer welcome here," said student leader Fadlallah Abu Wafia.
Protesters unfurled banners reading "Shame on USA" and "Vive la France, Arab governments go to hell".
In the Syrian capital, Damascus, demonstrators waved Iraqi flags and denounced US President George W Bush as a war criminal.
Crowds tried to advance on the American embassy before being forced back.
They also marched on the Egyptian embassy, and the embassy of Qatar - the host of the US Central Command for the duration of the war.
Police in the Jordanian capital Amman used force to break up an anti-war march by lawyers belonging to the Jordanian Bar Association.
At least four of the lawyers were injured, needing hospital treatment.
Turkish protestors marched through central Ankara, where the parliament has voted to allow US planes to use Turkish airspace for the war on Iraq.
Children on the march
Some of the biggest outbursts of opposition to the war have happened in Europe.
At least 100,000 people marched through the Greek capital, Athens, many of them high school students.
"We left class and asked our professors to join us," said Lefteris Faniotakis, a 15-year old student heading a group of 100 schoolmates.
Tens of thousands of school and university students in Italy staged spontaneous rallies in towns and cities all over the country.
Eggs were thrown at the British consulate in Venice and police used teargas to disperse demonstrators.
More students staged a sit-in outside Nato headquarters in Naples.
Germany, France and Britain also saw big demonstrations, many involving schoolchildren.
There have also been widespread protests in Asia.
In Indonesia, 2,000 people took part in a march in Jakarta in support of the government's call for an end to the war.
And many anti-war demonstrations have been held in Australia - which has committed warships, aircraft and 2,000 troops to the conflict.