The event, organised by the Palestine Solidarity Campaign, began at Russell Square in the city centre before attendees marched to Trafalgar Square for a mid-afternoon rally.
A smaller number of demonstrators also turned out for a counter-protest in support of Israel, with lines of police separating the two gatherings.
London has seen numerous large-scale pro-Palestinian protests since Israel mounted its military response to Hamas’s unprecedented attack on October 7, as well as smaller pro-Israel counter-demonstrations.
The rallies for Palestinians have proved contentious, attracting criticism for fuelling a hostile environment towards Jewish people. Some ruling Conservative lawmakers have branded them “hate marches”.
Police have made dozens of arrests for anti-Semitic chanting and banners, promoting a proscribed organisation and assaulting emergency workers.
But organisers insist they are exercising their democratic rights and that law-breakers are a tiny minority of the sometimes tens of thousands of people who turned out.
Sally Worgan, 65, a retired social worker, had travelled from Gloucestershire in western England to attend.
“I think it’s important for the Palestinian people to know that people do support them, that they’re not just on their own,” she told AFP.
“We will keep marching,” Ben Jamal, one of the organisers, told Sky News on Saturday, adding attendees wanted “a permanent ceasefire” and more aid to be delivered to Palestinians in Gaza.
“We already know that 70 percent of the British public according to opinion polls support the demands that we’re making.”
By mid-afternoon, London’s Metropolitan Police said officers had arrested one man “on suspicion of a terrorism-related offence in relation to inviting support for a proscribed organisation”.
The force had said in the build-up to Saturday that it had “a robust policing plan in place”, including requiring marchers keep to a pre-approved route and disperse by 5:00 pm (1700 GMT).
“Anyone who fails to comply with the conditions will be dealt with by officers,” the force had added on social media.
Prime Minister Rishi Sunak earlier this month called for officers to take tougher action against protesters at such events, saying they “had descended into intimidation, threats and planned acts of violence”.
jj/imm