The snap general election called by Theresa May is Britain's chance to avoid a "disastrous hard Brexit", the Liberal Democrats have said.
Party leader Tim Farron told voters "this is your chance to change the direction of your country" after the Prime Minister called an early vote on June 8.
The Lib Dems have consistently called for a second referendum on the final Brexit deal Mrs May achieves after negotiations with the European Union, and want to keep the UK in the European single market.
The Tories' 2015 general election victory saw the party gain several formerly Lib Dem seats in the south-west of England.
Mr Farron said: "This election is your chance to change the direction of our country.
"If you want to avoid a disastrous hard Brexit.
"If you want to keep Britain in the single market. If you want a Britain that is open, tolerant and united, this is your chance.
"Only the Liberal Democrats can prevent a Conservative majority."
Green Party co-leader Caroline Lucas welcomed an early vote at a time when "Britain is at a crossroads" and said the people should be given a say over the country's future direction.
She promised the Greens would present a "bold, positive vision for a different kind of Britain".
"At this election we will stand for an economy that works for everyone, not just the privileged few; a Britain that's open to the world and the protection of our precious environment," Ms Lucas said.
"We will stand up to the politics of hatred and division that is scarring our communities and give people across the country a chance to vote for a better Britain."
Her co-leader, Jonathan Bartley, said the announcement of a snap election, following a string of denials there would be an early vote, revealed a "deep dishonesty at the heart of government".
Professor Martin Smith, head of the University of York's politics department, said: "There is a danger that the Prime Minister's decision to call a snap election may appear both cynical and hypocritical.
"She had indicated several times that she would not do so and in order to do so she has to shift away from fixed term elections introduced by her predecessor.
"Clearly the reason for the election is that with Labour so weak in the polls, it is an opportunity for the Conservatives to win a landslide victory.
"With a large Conservative majority, the government will be able to get through any Brexit deal.
"Of course, there are going to be several difficulties for the Conservatives in the election campaign including the impact on Scotland and the potential for a second referendum, and a focus in the campaign on what sort of post-EU Britain the Conservatives want."
Ukip leader Paul Nuttall said: "We welcome the opportunity to take Ukip's positive message to the country.
"However, we believe that the Prime Minister's decision to call this election is a cynical decision driven more by the weakness of Corbyn's Labour Party rather than the good of the country.
"There is also the prospect of a slew of Tory held by-elections caused by the seeming systematic breach of electoral law at the last election, predominantly in places where Ukip were pressing the Conservatives hard.
"We are in the midst of Brexit negotiations so this election will provide a perfect opportunity for the 52% to vote for Ukip, the only party wholeheartedly committed to a clean quick and efficient Brexit."