People begen gathering in Mayfair’s Grosvenor Square from 6pm. More than a thousand people said they were interested in attending on the event’s Facebook page.
Several hundred people holding signs saying “no to racism. No To Trump”, and “refugees welcome”.
Stand Up To Racism, the group organising the rally, said it was prompted by a number of incidents which have seen the Republican accused of fear-mongering and racism, including his comments on Mexican people, condoning the beating of a Black Lives Matter activist and criticising the mother of a Muslim US Army soldier who was killed in Iraq.
Sabby Dhalu, a co-convenor at Stand up to Racism, said: “Donald Trump used the oldest trick in the book - he stirred up fear and racism in the context of a stagnant economy and the resulting fall in living standards - to mobilise a vote for him.
“Racism and sexism have been the hallmarks of Trump’s campaign. Trump failed to disavow the Klu Klux Klan after it pledged support for him.
“The danger now is racists across the globe feel emboldened by Donald Trump’s victory, and racism and sexism become normalised through the most powerful figure in the world.
“We call on anti-racists and all progressives to join us tonight outside the US embassy.”
Weyman Bennett, also a Stand up to Racism co-convenor, said: “It’s vital that all black and white people come together and oppose the politics of racism and division that Donald Trump represents.
“We must stand in the traditions of the Civil Right movement and begin the anti-racist fight back. In Britain a new anti-racist movement is born.”
In his victory speech, Trump pledged to work for the “forgotten” people of America.
He said: “I pledge to every citizen of our land that I will be president for all Americans, and this is so important for me.
“For those who have chosen not to support me, I’m reaching out to you for your guidance and your help so that we can work together to unify our great country.
“The forgotten people of our country will be forgotten no more.”