Participants in the "National Rejoin March" gathered near Hyde Park where they drove towards Parliament Square as pro-prop drivers honked their horns.
Demonstrators held banners reading "The road to rejoining the EU starts here" and "Rejoin and rejoice".
The National March for Readmission supports Britain's return to the European Union, which it left after the 2016 referendum.
During the tenure of former Prime Minister Boris Johnson, the Brexit deal, which came into force in 2021, was negotiated, with a date for review in 2025.
Peter Kaur, co-founder of the National Rejoin March, said he decided to organise the rally because he "felt like everyone had given up" of the cause.
Kaur, a truck driver from the central city of Derby, told local agency PA: "Brexit has been a huge mistake, and all of us, especially the working class and the poor, are paying for it, and we have to do something about it."
Opposition Labour Party leader Keir Starmer on Friday denied he wanted Britain to return to the European Union after his latest comments prompted the ruling Conservative Party to accuse him of wanting to "cancel Brexit".
Starmer insisted there was no reason to return to the EU, its customs union or the single market.
Ed Davey, leader of the Liberal Democrats, the fourth-largest party in Britain's parliament that has previously vowed to "stop Brexit", said voters had stopped talking about Europe on the doorstep, adding ahead of his party's annual conference on Saturday: "We want Britain to return to the heart of Europe but we are also realistic that it will take time."