Sarah Montgomery and Claudio Solano, from North London, arrived in white protective face masks that read “Trump stinks”.
They said they had had the masks since Trump’s first state visit.
“He still stinks,” Sarah says. “Everything he stands for stinks.”
Mr Solano, from Brazil, said he disagreed with both Trump’s policies and his support for Brazil’s former right-wing leader Jair Bolsonaro.
“It is disgusting, and I don’t understand why Britain has to be so cowardly,” he added.
As the crowd moved, it paused outside Downing Street, which was being protected by officers from Wales and Northern Ireland.
Some of the demonstrators carried signs including “Duck Off Donald” and “Feed him to the Corgis”.
At Parliament Square – where the march stopped – a number of people took to a stage, starting off with a performance by singer Billy Bragg.
Other speakers expected to make an appearance include former Labour MPs Jeremy Corbyn and Zarah Sultana, comedian Nish Kumar, and Green Party leader Zack Polanski.