More than 30 organisations, including the British Muslim Initiative and the Stop the War Coalition, have worked together to organise the series of protests.
The BBC's Barnie Choudhury estimated there were 7,000 to 10,000 people marching along the Embankment in central London towards Trafalgar Square. Some chanted "Free, free Palestine" and "Israel terrorists".
On Whitehall, hundreds of 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. A firework was also reportedly thrown at police.
Elsewhere, an estimated 2,000 people marched through Manchester and in Portsmouth, nearly 500 people took to the streets. Police said there were about 500 demonstrators in Glasgow and 600 in Edinburgh.
Former model Bianca Jagger and singer Annie Lennox have supported the action, and have also called on American president-elect Barack Obama to speak up against the bombardment.
Ms Lennox told the BBC that both sides were "wrong" and a total ceasefire was the only sensible solution.
She said the intervention of President George W Bush, who has described Hamas's rocket attacks as an "act of terror", was not helping the situation.
She said: "The problem is, from my perspective, they are pouring petrol onto the fire.
"They have to sit down. This is a small window of opportunity just before things kick off.
"For every one person killed in Gaza, they are creating 100 suicide bombers. It's not just about Gaza, it's about all of us.
Actor Paul Kaye, whose mother-in-law was killed in a Hamas rocket attack, told the BBC he had experienced the situation from an Israeli point of view.
"It's terrifying," he said. "My wife was trapped in a supermarket in a rocket attack. I was with my sons in Ashkelon station, holding them, waiting for a rocket to land and shutting my eyes thinking 'is this it?'
"So I think it's important to remember that mothers fleeing rocket attacks holding their children have the same fear on both sides of the border."
The UN has reported that some 2,000 Palestinians have been wounded since the airstrikes began last Saturday.
Thousands marched past Downing Street in protest at the violence
More than 400 people have been killed including 60 civilians - 34 of them children.
Four Israelis - three civilians and one soldier - have been killed by rockets fired into Israel from Gaza, which have hit towns up to 25 miles (40km) from the narrow coastal strip.
Both sides have so far resisted international calls for a ceasefire.
Foreign Office Minister Bill Rammell said the British government was "forcefully and strongly arguing" for an immediate end to the violence.
"Israel has a right to respond proportionately, but we've been very, very clear that there has been a massive loss of innocent life... and that is unacceptable."
Mr Rammell said Hamas rocket attacks on Israel were also "unacceptable", but stressed that "a reinvigorated political process" was the only route to a long-term solution.
Liberal Democrat leader Nick Clegg branded Israel's strategy of bombing Gaza "self-defeating".
"One of the great victims of the Israeli action is... moderate public opinion in the Arab world, upon whom Israeli long-term security interests depend," he said.
He said the reaction of the EU had been "weak" and said member states should have threatened to sever trade links with Israel if the bombardment continued.
Chairwoman of the Jewish Labour Movement, MP Louise Ellman, defended Israel's actions.
She told the BBC: "The scale of human suffering of the people in Gaza is deeply distressing but the responsibility lies with Hamas."
She described the militant group as an "organisation that does not accept the existence of Israel" and one that "refuses to be involved in negotiations" that would result in a two-state solution in the region.
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