Tim Farron is expected to deliver a rallying call to Remainers in Westminster today
Anywhere between 25,000 and 100,000 demontstrators are expected to join Saturday's Unite for Europe march, starting at 11am outside the Hilton Hotel in Park Lane.
From there the protestors will head through Trafalger Square and down past Downing Street, ending with a rally in Parliament Square – where hundres of floral tributes to the victims of Wednesday's attack have been laid.
Lib Dem leader Mr Farron is expected to insist that "democracy continues" in light of this week's atrocities.
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.
It is the latest protest calling for Britain to remain in the European Union, and comes 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.
Four people died in Wednesday's terror attack
We will march on the heart of our democracy and reclaim our streets in honour and respect of those that fell
Statement from Unite for Europe organisers
"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."
Get Quotes on Home Insurance
Mr Farron will use the rally to renew his call for a second referendum to "change the direction of our nation".
Floral tributes in Parliament Square will greet demonstrators today
He is expected to say: "The choice is who should decide the final deal.
"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.
Remainers demonstrating in Parliament Squre on the night the Brexit Bill was finally pushed through
"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.
The march also coincides with the EU's 60th anniversary celebrations in Rome, where leaders of the other 27 member states will gather to discuss plans for the future of the union without the UK.
Yesterday anti-EU protestors held a protest in the Italian capital, displaying banners proclaiming 'F*** EU" and giving speeches against Brussels.