Extra police officers were in place today as thousands of people gathered to stage a protest against Brexit.
The Unite for Europe march is to end with a rally in Parliament Square, the scene of floral tributes to victims of Wednesday’s terror attack.
Between 25,000 and 100,000 demonstrators are estimated to be attending, joining calls for Britain to remain part of the European Union.
The Metropolitan Police has announced more officers will be on duty throughout the march from Park Lane to Parliament Square.
Its events team tweeted: "From 11am today there will be a #UniteForEurope march from Park Lane to Parliament Sq. Road closures will be in place around #Westminster.
"In Central #London today there will be additional Police officers on duty for security and reassurance."
The protest comes just days before Theresa May triggers Article 50 to begin the exit process on March 29.
The march on Westminster comes also just days after knife-wielding terrorist Khalid Masood attacked the heart of British democracy.
As protesters travel along Piccadilly, Pall Mall and Whitehall, pictures of the mass gathering began to emerge online.
Pro-remain banners and European flags were seen among the crowds.
Some of the Unite for Europe marchers were seen carrying yellow flowers that they will lay at the memorial for the victims of Wednesday’s attack.
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."
Liberal Democrat leader Tim Farron was due to 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 was 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.