Tens of thousands of anti-Brexit demonstrators fell silent at Parliament Square in tribute to the victims of the Westminster attack.
Unite for Europe campaigners marched through central London to Westminster, the scene of floral tributes to those killed and injured in Wednesday's atrocity.
Opening the event, Alastair Campbell said: "Before we talk about Brexit, before we call on any of the speakers, we need to recognise that something really bad happened not far from here just the other day."
Campaigners stood with their heads bowed for a minute-long silence on Saturday, with the only sound the chiming of Big Ben.
Between 25,000 and 100,000 demonstrators attended the event, calling for Britain to remain in the European Union - just days before Theresa May triggers Article 50 to begin the exit process on March 29.
The sunny square was filled with protesters, many draped in the European flag and waving banners aloft, including a number declaring: "We are not afraid". Another sign read: "I'm 15 I want my future back!"
Liberal Democrat leader Tim Farron addressed the crowd, insisting "democracy continues" and adding: "We stand in defiance of that attack."
Lib Dem former deputy prime minister Nick Clegg, Labour MP David Lammy and Green co-leader Jonathan Bartley were also among those addressing the crowd.
Organisers refused to call off the event after the attack earlier this week, saying 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."
The march 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.