A separate march organised by the Palestine Coalition was due to join the route before both groups converged at Whitehall, where a video message from London Mayor Sir Sadiq Khan and speeches, including by Green Party leader Zack Polanski, were expected to be heard.
Several other politicians were also in attendance, including Your Party MP Jeremy Corbyn and Labour’s Diane Abbott.
Police imposed conditions preventing the joint demonstration continuing beyond 17:00 GMT.
Among those attending was Steve Tribble, who travelled from Stroud with what he described as a “radical left-wing band” of musicians. He said he felt compelled to join the demonstration because of growing concerns about the far-right.
“I understand that populism is spreading all over the world and that people are trying to look for scapegoats, they’re angry,” he said. “But we’re worried, that’s why we’re here.”