"Free Palestine. Let Gaza live!" they chanted [in Washington] as they marched. Some banners called on incoming President-elect Barack Obama to change U.S. policy.
"My country gives billions of dollars a year to support a disgusting regime that practices genocide," said Washington resident Mary Carrick.
Police in riot gear confronted around 20,000 protesters brandishing banners and Palestinian flags outside the Israeli embassy in central London, while Oslo police used tear gas against rock-throwing activists in the Norwegian capital.
About 30,000 took to the streets of Paris, many demonstrators wearing Palestinian keffiyah headscarves and chanting "we are all Palestinians", "Israel murderer" and "peace". Some threw stones at police and burnt Israeli flags.
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More than 40,000 people protested against the Gaza assault in towns across Germany, while demonstrators at the Israeli embassy in Dublin threw shoes and carried a mock coffin, covered with pictures of wounded or dead Palestinian children.
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In Athens, hundreds of riot police carrying plastic shields watched as about 2000 demonstrators marched to the Israeli embassy, fearing a revival of violent street protests which rocked the city last month.
Thousands marched in Sweden's biggest cities, burning Israeli flags and setting off fireworks.
Protesters gathered in several Italian cities, some shouting "Free Palestine, Gaza will win" while others burnt an Israeli flag. Protests in Poland were largely peaceful.