Violent clashes broke out near the Israeli embassy in London, when thousands of demonstrators staged a rally demanding an immediate ceasefire in Gaza, leading to the arrest of 15 protesters.
Helmeted riot police with batons and shields charged a group of demonstrators, who hurled sticks, shoes and traffic cones back at them while chanting "Free Palestine" on Saturday.
Protesters tried to force their entry to the North gate of Kensington's Palace Gardens and six climbed an adjoining wall, setting fire to an American flag.
The windows of a Starbucks opposite the embassy were smashed. Some protesters also attempted to throw barriers and other missiles at police.
The violence started when police tried to move protesters away from the gates of the Israeli embassy. One police officer was knocked unconscious and two more were injured.
Fifteen protesters were also arrested, media reports said.
Many people were pleading with police to allow them out of the cordon as they were trapped between two lines of officers.
There were running skirmishes between the officers and groups of young men, each time prompting a further charge from police and sending the crowd running screaming in the opposite direction.
In Edinburgh, three police officers suffered minor injuries.
Scotland Yard's commander Bob Broadhurst said: "A group of people on this demonstration have set out to deliberately confront and antagonise police officers trying to protect the embassy of Israel.
"We are very disappointed by the irresponsible and criminal actions of those who have challenged police by ripping apart security barriers and throwing objects at them. A hard core of demonstrators are undermining the cause of the vast majority of people on this demonstration."
Organisers brought together a number of different groups including the 'Stop The War Coalition', the British Muslim Initiative and the Palestine Solidarity Campaign.
Nick Napier, a protest organiser, said the action was taken as a result of the "rage and anger" over the death toll in Gaza over the past two weeks.
"People are here because they know the trail of blood leads from Gaza back to Britain and (prime minister) Gordon Brown. It's another year, another war, another massacre. It's a war too many for most people," he said.
Celebrities including Brian Eno and Annie Lennox spoke to the 20,000-strong crowd, which marched from Speaker's Corner in Hyde Park to the Embassy in Kensington.
Protests also took place in Aberdeen, Newcastle, Manchester and Southampton.
