Several thousand people crowded the streets outside the Israeli Embassy in west London, waving placards that read "Gaza: End the Siege" and "Freedom for Palestine."
The crowds blocked entire streets, but police said the protest was largely peaceful.
A smaller protest took place in Paris today, where about 100 people demonstrated near the French Foreign Ministry.
Demonstrators waved Palestinian flags. One woman shouting "Palestine will survive" had the Palestinian flag painted on her cheeks.