Tens of thousands of people have marched in central London to demand a final vote on any UK exit deal, on the second anniversary of the Brexit vote.
Organisers of the People's Vote march said Brexit was "not a done deal" and people must "make their voices heard".
Meanwhile, hundreds attended a pro-Brexit counter-protest.
It came as senior Cabinet ministers, including Liam Fox and David Davis, insisted the UK is prepared to walk away from talks without an agreement.
The protest is part of a "summer of action" by campaign groups designed to increase pressure on Theresa May and Jeremy Corbyn.
Organisers said at least 100,000 people attended, but police have not provided an official estimate.
World War Two veteran Stephen Goodall, 96, led the pro-EU protesters as they headed from Pall Mall to Parliament Square.
There were boos from the crowd as the march approached Downing Street. After showing anger towards the PM, some began to chant, "Where's Jeremy Corbyn?"
Among those addressing the demonstrators was Gina Miller, who successfully campaigned to ensure the UK could not trigger talks on leaving without the approval of Parliament.
She said: "Together we must stand up, demand our voices are heard, demand a people's vote so that future generations can hear us say we did our bit we stood up and shouted for a country that's together, kinder, tolerant.
"This is not a time to be silent."
Liberal Democrat leader Sir Vince Cable said Brexit was "not a done deal" and could be reversed, while Green Party co-leader Caroline Lucas told the crowd that Brexit "will be a disaster for this country".
One of the rally organisers, James McGrory from pressure group Open Britain, said there should be "a choice between leaving with the deal that the government negotiates, or staying in the European Union".
Britain is due to leave on 29 March 2019, 46 years after it first joined the European Economic Community, the forerunner to the EU.
The government is giving Parliament a vote on the final deal, if one is reached, in the autumn - but it remains unclear what will happen if they reject it.