Organisers of the march say “immediate action” is needed by the UK Government to help end the “brutal” violence against the Palestinian people.
Huge crowds then gathered outside the gates of the embassy in Kensington, calling for an end to air strikes on the territory by the country’s armed forces.
A temporary stage had been set up on Kensington High Street and organisers urged people to keep moving down the road as numbers continued to swell.
Organisers said crowds stretched back to Bayswater Road, with around 150,000 people participating.
Footage was posted on social media, as the hashtags “Marbel Arch” and “Free Palestine” trended on Twitter:
Multiple speakers adressed the throng outside the Israeli embassy, including former shadow home secretary Diane Abbott and ex-Labour leader Jeremy Corbyn.
Crowds chanted: “Oh, Jeremy Corbyn,” and threw roses as he took to the stage.
“Think what it’s like being a mother or father and seeing a building bombed in front of you, knowing your family is in there, and you can do nothing,” the MP said.
“It’s our global voices that will give succour, comfort and support in those settlements alongside Gaza and all over the West Bank, East Jerusalem who are suffering at this time.
“End the occupation now. End all the settlements now and withdraw then. End the siege of Gaza now.”
Husam Zumlot, Palestinian ambassador to the UK, said: “This time is different. This time we will not be denied any more. We are united. We have had enough of oppression.
“Today we are saying enough, enough with the complicity. Thank-you for standing with us.”
Abbott then told those present: “We must remember we are part of an international movement. This is a worldwide movement for justice.
“Palestinian people are having their land seized… and they are now being killed in their homes.
“All of this is illegal.”
Other speakers outside the Israeli embassy were Labour MP Zarah Sultana and rapper Lowkey.
The names and ages of the children killed in the conflict were read out and a minute’s silence was then held.
There were cheers as organisers told them the turnout was “the biggest pro-Palestine demonstration since 2014”.
Elsewhere, smoke was set off along Kensington High Street and some demonstrators climbed on to buildings and bus stops.
Responding to the incident, Metropolitan Police said: “Officers are engaging with a group of people who have gathered for a demonstration in central London this afternoon.
“A policing plan is in place to ensure everybody is kept safe and to reduce the spread of Covid-19.”
It comes amid reports that an Israeli air strike destroyed a high-rise building that housed the Associated Press (AP), Al-Jazeera and other media groups in the Gaza Strip.
The AP said the air strike came roughly an hour after the Israeli military ordered people to evacuate the building.
The news agency said there was no immediate explanation as to why the building was targeted.
Thousands of Palestinians have been forced to flee from their homes after a week of sustained conflict.
Since Monday night, Palestinian militant group Hamas has fired hundreds of rockets into Israel, whose military responded by barraging the Gaza Strip with tank fire and air strikes.
At least 126 people have been killed in Gaza, including 31 children and 20 women.
In Israel seven people have been killed, including a six-year-old boy and a soldier.
The demonstration in London was organised by Palestine Solidarity Campaign, Friends of Al-Aqsa, Palestinian Forum in Britain, Stop The War Coalition, Campaign for Nuclear Disarmament and the Muslim Association of Britain.
A spokesperson for the organisers said: “It is vital that the UK Government takes immediate action.
“It must stop allowing Israel’s brutal violence against and oppression of the Palestinian people to go unpunished.
“The bombardment of Gaza which is killing civilians including children is a war crime.
“The UK Government is complicit in these acts as long as it continues to offer Israel military, diplomatic and financial support.”