The anti-Brexit pledges at the heart of the manifesto are meant to position the Lib Dems as the party of hard remain.
May has vowed to press ahead with Brexit, saying she was prepared to take Britain out of the European Union even if no deal with the bloc could be struck, while Labour's leader Jeremy Corbyn has said the issue has been settled.
"You should have your say on the Brexit deal and if you don't like the deal, you should be able to reject it and choose to remain in Europe", he said.
Former minister Vince Cable deemed the ruling Conservatives' approach "very foolish", as he told AFP of the party's election chances.
The Lib Dems would assemble a constitutional convention to "deliver home rule to each of the nations of a strong, federal United Kingdom".
"It is going to wreck our children's future for years to come", he said.
However, recent polling from Sky Data shows there has not been a significant shift in support towards the party, with only 61% of supporters who backed them in 2015 doing so again this time.
This would also lead to "single place-based budgets" for health and social care services by 2020, which the party claimed would allow "local areas to decide how best to provide the full spectrum of care for their community".
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"Not Theresa May's cold, mean-spirited Britain".
Mrs May said the Tory manifesto out later this week will "set out in detail the five great challenges our country faces over the next few years and lay out how we will tackle them", compared to the "fantasy wish list of easy promises paid for with imaginary money" offered by Labour.
The manifesto also outlines proposals to restore housing benefit for young people, create a discounted bus pass for 16-21 year olds and introduce votes for 16-year-olds at elections and referendums across the UK.
If elected, they would introduce a "rent to own" scheme to help first time buyers purchase a house over 30 years.
Key pledges include plans to increase education spending by £7bn, scrap the planned extension of grammar schools and extend free school meals to all primary school pupils. "Leaders of political parties tend to stand down if their general election has not been successful".
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".
"(We're) playing the long game, we're trying to make sure we have an effective base as an opposition", said the veteran Lib Dem politician.