Police have dispersed crowds of demonstrators who gathered near the Israeli embassy in London, following a day of protests across the UK.
The Metropolitan Police said between 10,000 and 12,000 people attended the march, but organisers claimed the figure was as high as 75,000.
Up to 5,000 protesters were involved in a stand-off with police in Kensington, leading to 15 people being arrested.
Foreign Secretary David Miliband said an urgent ceasefire was needed.
The demonstration in London was one of many in cities around the world.
As the protest continued at the embassy, it emerged that Israeli ground troops had entered Gaza.
Protesters burned Israeli flags and threw missiles at police
The Israeli government has said it is defending its citizens from Palestinian rocket attacks, but the British government has repeatedly called for a ceasefire.
Mr Miliband said Israel's ground incursion would cause "alarm and dismay", but "intensive diplomatic efforts" were continuing to find a solution.
Shadow foreign minister David Lidington said the ground invasion was "a serious development that is bound to lead to yet more loss of life".
In London, crowds marched along the Embankment towards Trafalgar Square.
People were crying, they were very, very upset
Protester Stephen Hodgkins
When they reached Whitehall about a thousand shoes were thrown at the gates of Downing Street, echoing the protest of an Iraqi journalist who threw his shoes at US President George W Bush.
Later, a group of protesters broke off from the main group and headed for the Israeli embassy.
Roads were closed and police in riot gear used barriers to contain the crowds on Kensington High Street and away from the embassy itself.
Police said a small number of people had made repeated attempts to break through the barriers and threw sticks and other missiles at officers, before pulling apart the barrier line throwing them at the police.
The organisers of the demonstration said they would make an official complaint to Scotland Yard alleging riot officers charged into protesters as they moved through the Hyde Park underpass towards the embassy.
Some 600 people, young and old, marched through Edinburgh
Chris Nineham, from the Stop The War Coalition, claimed police had charged at least three times, wielding batons and injuring some demonstrators.
One protester Stephen Hodgkins, 38, also said police had charged several times.
"People were crying, they were very, very upset. It was just shocking," he said.
Scotland Yard said police had made "one 10-metre advance into the crowd to regain control of the protest, using recognised and proportionate tactics".
Earlier former London mayor Ken Livingstone addressed the crowds in Trafalgar Square.
He told the BBC the protests sent "a powerful message" to the prime minister that the public would not tolerate Israel's "war crime".
"The silence of the British government, and most Western governments, is obscene, and I think Gordon Brown will see this and recognise the real anger amongst Londoners," he said.
In Manchester, up to 2,000 people joined the demonstration
Also speaking in Trafalgar Square, veteran politician Tony Benn praised those who had turned out, and called for a Europe-wide boycott of Israeli goods.
Elsewhere, some 2,000 protesters joined a rally in Manchester. In Portsmouth, nearly 500 people took to the streets and some 300 marched in Bristol.
Police said there were about 500 demonstrators in Glasgow and 600 in Edinburgh, although organisers said there were more like 2,000 protesters in each city.
Comedian Alexei Sayle told the BBC Israel's response was "massively disproportionate" and although the two countries had a complicated history, the answer was simple.
He said: "Stop killing people, stop killing children and then start thinking about the little things after that."
But actor and comedian Paul Kaye said the situation was just as "terrifying" for Israelis under constant Hamas fire.
Actor Paul Kaye: Fear is the same on both sides of the border
Israel stepped up its offensive against Gaza on Saturday, using tanks and artillery to bombard the territory.
The UN says that since the start of the violence a week ago more than 400 people have been killed, about 25% of them civilians - including at least 34 children.
Four Israelis - three civilians and one soldier - have been killed by rockets fired into Israel from Gaza.
Both sides have so far resisted international calls for a ceasefire.
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