The march was one of the biggest pro-Palestinian marches held in London since 7 October. It was also attended by a group of Jewish protesters.
In Irish capital Dublin, tens of thousands of people gathered to call for action on Gaza and an immediate ceasefire.
Spanish capital Madrid saw thousands of protesters marching for an end to bloodshed in Gaza.
In German city Munich, where world leaders and ministers are attending the Munich Security Conference, protesters gathered some 200 meters away from the main conference venue to call for a ceasefire.
Tens of thousands of protesters also filled the main Dam Square in Dutch capital Amsterdam.
“Ceasefire Now”, “Stop the Genocide”, and “Free Palestine” read on many placards and banners carried by the crowds.
The second Global Day of Action was organised by the Palestine Coalition formed by Palestine Solidarity Campaign, Palestinian Forum in Britain, Stop the War Coalition, Friends of Al-Aqsa and Muslim Association of Britain.
“Over 1.7 million Gazans have been forcibly displaced from their homes, more than 28,000 Palestinians have been killed and another 100,000 injured, in what the ICJ has accepted as a plausible case of genocide,” a joint statement from the coalition said.
“Despite the ICJ calling on Israel to stop genocidal acts the Israeli Government has made clear it intends to proceed with a full scale military assault on Rafah,” it added, referring to Israeli plans to attack Rafah.