People's Vote, which wants a referendum on any exit deal, said the public must make their "voices heard" about the damage of leaving next year without agreement.
As it stands, the UK is due to leave on March 29, 2019, 46 years after it first joined the European Economic Community, the forerunner to the EU.
But the People's Vote campaign says this should only happen if the withdrawal deal negotiated by Prime Minister Theresa May and the other 27 EU members is approved in another public vote.
Saturday's demo, which will begin in Pall Mall and culminate outside the Houses of Parliament, is part of a "summer of action" by campaign groups designed to increase pressure on May and opposition Labour Party Jeremy Corbyn.
May and Corbyn have rejected calls for another public vote, saying the will of the people expressed in the 2016 ballot is clear, although many Labour MPs now want another referendum.
Organisers say people "from all walks of life" will be present, including farmers, doctors, teachers, delivery drivers, students, fishermen and veterans, demonstrating the "growing popular demand" for another vote.
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