Tim Farron has lumped Theresa May in with far-right figures like Donald Trump and Marine Le Pen as he accused the Prime Minister of allowing the Conservatives to be "taken over" by an agenda forged by Nigel Farage.
Mrs May's Ukip-inspired "hard Brexit" approach of taking Britain out of the single market and customs union is "a time bomb under our economy" which will wreck Britain's future for decades to come, warned the Liberal Democrat leader.
Launching the Liberal Democrat manifesto for the June 8 General Election, Mr Farron said the document set out a vision of an "open, tolerant and united" Britain far removed from the "cold, mean-spirited" country favoured by Mrs May and Mr Farage.
In the face of opinion polls suggesting the Lib Dems are failing to make a breakthrough, Mr Farron urged Britons to follow the lead of French voters who rejected the "two tired old parties" to elect President Emmanuel Macron.
The Lib Dem manifesto set out plans to fight hard Brexit, ban diesel cars and help young people buy their first home.
The party promised to pump an extra £6 billion a year into health and social care through a 1p rise in income tax and raise £1 billion in revenue by legalising cannabis.
But it said it will not scrap university tuition fees, insisting the NHS is a bigger spending priority.
Mr Farron has already said he expects a Tory landslide on June 8. But speaking in east London, he said the more Lib Dem MPs are elected, the better chance Britain has of a good Brexit.
Mrs May's decision to pull Britain out of the single market was not on the ballot paper last year and any final Brexit deal should be subject to a second referendum, he said.
"That decision alone is a time bomb under our economy," warned Mr Farron. "And when it blows up it is going to take our NHS and our schools down with it. It is going to wreck our children's future for decades to come."
Mr Farron said the election's most revealing moment was a tweet from Mr Farage hailing the Prime Minister for "using the exact words and phrases I've been using for 20 years".
The former Ukip leader's world-view is "the same one that leads to Donald Trump banning Muslims and building a wall, the same one that Marine Le Pen tried to impose on the decent people of France", said Mr Farron.
He added: "Nigel Farage's vision for Britain is now Theresa May's. He has taken over the Conservative Party. Anti-Europe. Anti-refugees. Slashing funding to schools and hospitals.
"No wonder Ukip is standing down candidates and backing the Tories. After all, who needs Ukip if the Government is doing what they want anyway?"
Labour has "lost the right to call themselves the opposition" by failing to make a stand on Brexit, Mr Farron said.
And in an appeal to voters to support the Lib Dems, he said: "Theresa May and Nigel Farage's cold, mean-spirited Britain is not the Britain I love.
"The Britain I love is generous and compassionate. The Britain I love is one where we are decent to each other. The Britain I love is open, tolerant and united.
"If that is the Britain you love too, then this is the moment to stand up. This is your chance to change Britain's future."
While majoring on plans to fight hard Brexit, the Lib Dem manifesto also set out a package of more than £13 billion of tax rises to fund public services, including a penny on every income tax band and a rise to 20% for corporation tax.
The party plans to bring in a diesel scrappage scheme and ban the sale of diesel cars and small vans by 2025, while motorists will also face new levies in ultra-low emission zones in 10 towns and cities.
A rent-to-own scheme would see monthly property rents used like a mortgage to allow tenants to buy their homes over a 30-year period.
Lib Dems said they will restore housing benefit to young people, bring in bus passes for 16 to 21-year-olds and lower the voting age to 16 if they gain power.
The party has also pledged to boost education spending by £7 billion over five years, double the number of businesses that take apprenticeships and extend free school meals to all primary school pupils.
Conservative chairman Sir Patrick McLoughlin said: "This manifesto makes one thing abundantly clear: a vote for the Lib Dems is a vote to put Jeremy Corbyn in Downing Street.
"From increasing taxes to borrowing more - from putting our security at risk to scrapping Trident - these policies are an echo of Corbyn's manifesto we saw earlier this week."
Paul Johnson, director of economic think tank the Institute for Fiscal Studies, said the Lib Dem tax proposals were "much more modest" than those unveiled by Labour on Tuesday, and would involve Mr Farron's party "increasing spending more than they're going to increase taxes".
Former Lib Dem leader Nick Clegg praised Mr Farron's "passionate" manifesto speech, saying the election is about young people.
He told ITV News: "We are heading towards a future of a very uncompromising approach to Brexit that 75% of young people didn't want.
"That's a pretty extraordinary thing to do as a country. To hurtle towards a future that the people who have to inhabit the future don't want."
Mr Clegg said his successor had performed well, saying: "I think he is emerging in this election campaign as by far the most authentic and human of all the leaders."
Mr Farron launched the party's manifesto at a techno club and the stage floor was adorned with Union Jack and EU flags.