Thousands of demontstrators joined Saturday's Unite for Europe march, starting at outside the Hilton Hotel in Park Lane.
From there the protestors will marched 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.
The march coincided 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.
The Lib Dem leader walked on stage to whoops and cheers from the crowd, before paying tribute to the police and in particular Keith Palmer, the PC stabbed to death on Wednesday.
Mr Farron said: "We are defiant about two things.
"First of all we are defiant against those who would seek us to be afraid after what happened last Wednesday.
"Those people who hate us – they hate our democracy, our freedom, our way of life, our liberty, our love of country – they hate our peace.
"We need to make sure our response is not to have less of those things but to have more, so we stand in defiance in that attack."
He continued:"But we also stand in defiance because we do not buy the argument that democracy ended on the 23 June last year.
"If you were the Prime Minister, and you're respecting the result of a referendum that was very close to being a dead heat, then you would pursue a consensual Brexit."
He said Theresa May's decision to leave both the EU and the single market made former UKIP leader Nigel Farage "look like a moderate".
"Nigel Farage has speny the last 20 years going around Europe saying 'let's be more like Norway and Switzerland'.
"What are Norway and Switzerland? They are out of the EU, and in the single market."
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Tim Farron launched a scathing attack on Theresa May's Brexit plan
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
He said the deal being drawn up by Number 10 represented "the worst affront and assualt to democracy imaginable".
"You don't know what that deal will look like, I don't know what that deal will looks like, Therea May doesn't know what that deal will look like and David Davis hasn't got a clue."
After his last remark drew loud applause, Mr Farron twisted the knife by adding suggesting Guy Verhofstadt, the EU's chief Brexit negotiator, "will eat him [Mr Davis] for breakfast".
He called for the final sign off on the Brexit deal to be given to the people, and expressed solidarity and love to everyone who voted Leave.
"we respect that," he said. "We do not belive for one minute they signed up to that narrow world view Theresa May has chosen.
"She does not speak for the 52 per cent – she barely speaks for the five per cent."
Thousands join the March for Europe through London
Tim Farron is expected to deliver a rallying call to Remainers in Westminster today
The march is taking place days before Theresa May triggers Article 50
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.
"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.
Thousands of demonstrators have gathered in Park Lane for today's anti-Brexit march
Floral tributes around Parliament Square following Wednesday's terror attack
"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.
Yesterday anti-EU protestors held a protest in the Italian capital, displaying banners proclaiming 'F*** EU" and giving speeches against Brussels.