A protest in the Belgian capital that drew 30,000 turned violent as well, with demonstrators overturning cars and smashing shop windows. And in Manila, Philippines, policemen used shields to disperse students protesting outside the U.S. Embassy.
Israel launched its campaign in Gaza on Dec. 27 to stop rocket fire from the militant Palestinian group Hamas. Gaza health officials say nearly 870 Palestinians have been killed, roughly half of them civilians. Thirteen Israelis have also died.
Some 2,000 protesters in the Pakistani port city of Karachi burned U.S. flags and chanted anti-Israel slogans, and several hundred of them marched on the U.S. Consulate, senior police official Ameer Sheikh said.
"They were in a mood to attack," Sheikh said. "They were carrying bricks, stones and clubs."
A U.S. Embassy spokesman in Islamabad, Lou Fintor, said the protesters did not get close to the consulate, which was closed Sunday.
Washington provides a large amount of foreign aid to Israel as well as military and weapons assistance. Israeli military action is often perceived in the Muslim world as being financed and supported by the U.S. While Pakistan's government is a U.S. ally, anti-American sentiment is pervasive in the Muslim majority country.
In Spain, as many as 100,000 attended rallies in Madrid and the southwestern city of Seville, urging Israel to "stop the massacre in Gaza" and calling for peace initiatives. Spain's Foreign Minister Miguel Angel Moratinos will tour the Middle East starting Monday to promote solutions to the conflict.
In downtown Beirut, an estimated 2,500 Lebanese and Palestinians protested peacefully, waving Palestinian flags and calling on the international community to intervene in the Israeli attack.
A convoy of some 15 ambulances from an Islamic medical society sounded their sirens for 20 seconds in solidarity with Gaza medics. Some protesters set fire to a large Israeli flag, while children held bloody dolls representing Palestinian children killed in Gaza.
The death of children in the Gaza assault has become an enduring theme at protests.
Children carrying effigies of bloody babies headed the march attended by thousands in Brussels, which later turned violent before police intervened with water cannons and arrested 10 protesters. Belgian lawmaker Richard Miller told Le Soir newspaper that he was hit in the face by a stone thrown by a demonstrator.
In New York, thousands of supporters of Israel rallied near the United Nations, declaring the offensive an act of self-defense.
"For the last three and a half years, Israel's been bombarded daily by a number of rockets coming from Gaza," said Gov. David Paterson, one of a dozen elected officials who addressed the crowd that filled a block of 42nd Street. "The founding charter of Hamas calls for the obliteration of the state of Israel."
Across town at Times Square, about 150 pro-Palestinian demonstrators waved signs with pictures of dead and wounded children and chanted, "No justice, no peace! Israel out of the Middle East."
Scuffles broke out between supporters of both sides after someone stomped on a Palestinian flag at another demonstration few blocks away. Seven officers who intervened were hurt in the melee and 10 people were arrested, police said.
Jewish communities appeared divided on the Israeli operations. In London, thousands of people gathered at Trafalgar Square to support the action in Gaza, while anti-Israeli protesters held a counter-demonstration nearby. In Antwerp, Belgium, home to a large Hassidic Jewish community, some 800 people took part in a peaceful pro-Israel demonstration.
In a letter published in Britain's Observer newspaper Sunday, 11 leading British Jews urged Israel to end its Gaza campaign and negotiate a settlement for security reasons.
"We are concerned that rather than bringing security to Israel, a continued military offensive could strengthen extremists, destabilize the region and exacerbate tensions inside Israel with its one million Arab citizens," the letter said.
In Syria, as revolutionary songs blared from loudspeakers, demonstrators accused Arab leaders of being complicit in the Gaza assault. "Down, down with the Arab rulers, the collaborators," the crowd in Damascus shouted.
Separately, activists protesting the Israeli campaign were driving from Turkey to Syria in a convoy of 200 cars, and participants hoped Syrian protesters would join them at the border Monday, according to Nezir Dinler, an activist with the Istanbul-based Solidarity Foundation.
A few thousand people marched in largely peaceful pro-Palestinian rallies in the Italian cities of Rome, Naples and Verona. In Rome, municipal authorities were dispatched to erase graffiti — including Stars of David and swastikas — that had been scrawled on Jewish-owned stores and restaurants overnight.
Associated Press Writers Zeina Karam in Beirut, Lebanon, Aoife White in Brussels, Albert Aji in Damascus, Syria, Harold Heckle in Madrid, and Nicole Winfield in Rome contributed to this report.