Photos show supporters of the Palestine Solidarity Campaign (PSC) gathering at Russel Square before embarking on their march to Porticullis House.
Huge crowds of demonstrators, many carrying Palestinian flags and placards calling for solidary, were seen gathered on a sodden street.
The Met Police deployed 700 officers across the capital in anticipation of the rally and other potential demonstrations, including a rumoured Just Stop Oil interruption.
A planned pro-Israel counter-protest was reportedly cancelled and did not materialise.
Hundreds of demonstrators surrounded Downing Street and were warned to disperse or face arrest. One person had already been arrested, police said in an update.
PSC director Ben Jamal explained the purpose of the march: "We're here to show the new government that we're not going away and to demand they take decisive action to end UK complicity in Israel's genocide in Gaza and defend Palestinian rights."
Mr Jamal criticised Prime Minister Sir Keir Starmer's previous stance on the Gaza conflict, saying that he had instructed Labour MPs not to vote for a ceasefire. The PSC director emphasised the need to "ramp up" pressure on the new Labour administration.
"We're not going away," Mr Jamal added. "You need to make sure the UK ceases to be complicit in the oppression of the Palestinian people."
Sir Keir has included in the Labour manifesto that a Palestinian state would be supported.
“It’s very important we have a viable Palestinian state alongside a safe and secure Israel,” he said last month.
Islington North MP and former Labour leader Jeremy Corbyn is said to have joined the movement.
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Mr Corbyn, now an independent MP, said: “Palestine was on the ballot in this election - and I promise to stay true to my word to stand up for the Palestinian people.
“We said it to the Tories, and now we will say it to Labour: a government that sells arms to Israel is a government that is complicit in crimes against humanity. A change in government doesn’t change the facts that the people of Gaza are still being murdered in their sleep. And it doesn’t change the fact that there is only one path to a just and lasting peace: an end to the occupation of Palestine.”
The Met said that the counter protest was set to be static and in the Strand area.
Videographer Matt Capon tweeted that he had been to the scene of the pro-Israel march, a group identified by the Met as ‘Enough is Enough’, but said he had seen no sign of it. He added that the counter protest had cancelled “five hours ago”.
The Standard was unable to reach a representative for the group.
It is thought to be a different campaign to the left wing ‘Enough is Enough’ pressure group backed by Labour’s Zarah Sultana and RMT union boss Mick Lynch.
Mr Capon wrote: “I'm walking back down Southampton Row towards the Strand towards Holborn and more people are arriving. There's apparently no sign of the pro Israel counter-protest.”
Police had drawn up and publicised a defined route for the marchers to move along.
Superintendent Martin Kirby, who is leading policing in London this weekend, said: "As always the goal is to protect the right to peaceful protest while minimising the impact of these events, including on London's Jewish communities.
"Police do not have the power to ban protests unless there is a risk of serious disorder.”
The Met has been contacted for more information.
