Around 1,500 took part in the march and rally in the centre of Glasgow, including Labour MP Anas Sarwar, who said the people of Glasgow were standing "shoulder to shoulder with the people of Palestine". A protest was also held in Aberdeen.
Sarwar said there was a "real injustice" happening that the world could not ignore, adding: "All those who believe in peace, believe in justice, believe in fairness and equality must now find their voice,"
In London, up to 15,000 people marched through the capital from Downing Street to the Israeli embassy in Kensington. A police blockade stopped them from gaining access.
Organisers repeatedly told the crowds to spread out during the speeches over fears people were being crushed. Two children were pulled over a barrier close to the stage and one organiser said 10 people had fainted in the crowd.
Other demonstrations were also held in major cities including Belfast and Dublin, where around 3000 people marched in the city centre.
And thousands took to the streets in French cities to condemn the violence in Gaza, defying a ban imposed after demonstrators marched on two synagogues in Paris last weekend and clashed with riot police.
Yesterday's demonstrations in Paris led to fresh clashes with police, with reports of demonstrators launching projectiles at riot police, who responded by firing teargas canisters and stun grenades. Demonstrators also climbed on top of a building and burned an Israeli flag.
Protests were also held in a dozen other cities across France, ranging from Lille in the north to Marseille in the south of the country.