The Unite for Europe march will end with a rally in Parliament Square, the scene of floral tributes to victims of Wednesday's outrage, where Liberal Democrat leader Tim Farron will insist "democracy continues".
Mr Farron is expected to say: "The unspeakable outrage that happened in this city on Wednesday will not defeat us, or silence us or divide us.
"Democracy continues, free speech continues, our way of life continues. Terrorism will not win."
Organisers announced on Thursday that the march would go ahead after talks with the police and Greater London Authority.
Between 25,000 and 100,000 demonstrators are estimated to attend, joining calls for Britain to remain in the European Union - just days before Theresa May triggers Article 50 to begin the exit process on March 29.
Organisers said in a statement: "We will not be intimidated. We will stand in unity and solidarity. We will march on the heart of our democracy and reclaim our streets in honour and respect of those that fell.
"We will be observing a minute of silence and remembrance at the start of the rally. We would encourage all attendees to bring with them some symbol of respect and to act in the appropriate fashion on the day."
Mr Farron will use the rally to renew his call for a second referendum to "change the direction of our nation".
"The choice is who should decide the final deal," he is expected to say.
"Should it be politicians or the people? The Liberal Democrats say the people.
"We can turn the tide of populism and we can change the direction of our nation - liberals and progressives can and will win again.
"I am not prepared to accept that our country is inevitably to become meaner, smaller, poorer. If you believe in democracy then you accept defeat with good grace...and you keep on campaigning for a better Britain.
"Our job is to win hearts and minds over these coming months, to win support for a referendum on the deal, to change the direction of the debate and to change the direction of our country."
Lib Dem former deputy prime minister Nick Clegg, Labour MP David Lammy and Green co-leader Jonathan Bartley are also expected to address the rally.