Tens of thousands of people joined a procession from Hyde Park to the Israeli Embassy, but a small number caused damage to street lighting and a gate as they tried to force their way into Kensington Palace Gardens.
Police reported coming under attack from a minority of protesters throwing sticks and stones. In Notting Hill, shoes have been thrown at officers on horseback.
In Edinburgh, where thousands have gathered, demonstrators have thrown shoes at the US Consulate.
Actress Lauren Booth, who is taking part in the London demonstration, criticised her brother-in-law Tony Blair, saying that his suggestions for a ceasefire in
Gaza would condemn Palestinians "to a slow agonising death".
Cherie Blair's half-sister said: "Tony Blair's only comment regarding the ceasefire has been to say that it can only take place after the tunnels in Gaza are destroyed.
"What he is suggesting means that after the massacre people will have no access to food, kerosene and medicines that came through those tunnels. That is not a ceasefire that is a slow agonising death."
Police say another group of protesters have occuped an Israeli cosmetics shop in central London.
In Belfast 5,000 demonstrators marched through the city centre and in Newcastle 100 people gathered to call for an end to the fighting.
Tomorrow morning the
Board of Deputies of British Jews will hold a rally calling for peace in Israel and Gaza in Trafalgar Square in central London.
And inManchester, 3,000 people are expected to gather for a pro-Israel demonstration.
Protests against the ongoing fighting have been taking place around the world.
Thirty thousand people marched through Paris shouting "We are all the children of Gaza" and in Lille, northern France, 10,000 gathered to protest.
In Algiers, 63 people were injured after police clashed with demonstrators.