Farron, who has admitted he wants to reverse the triggering of Article 50 and keep free movement from the European Union into the UK, 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."
Theresa May pre-empted the Lib Dem attack by saying the Liberal Democrats wanted to "grind the business of government to a standstill, prop up Labour and re-open Brexit debate".
According to the Liberal Democrat press office, 1,000 new members have joined the party since Theresa May's announcement earlier today.
And now Farron, the MP for Westmorland and Lonsdale, has sent a group email to their supporters, asking them to help block the Brexit process.
Politicians react as Theresa May calls snap general election on June 8 Tue, April 18, 2017
UK Prime Minister Theresa May has announced her intention to hold an early general election on June 8
AFP/Getty Images 1 of 5
Theresa May has called for a snap general election
Tim Farron said you must vote for him if you want to stop a hard Brexit
It reads: "Friend, this is it. Theresa May has called a snap General Election.
"This is our chance to change the direction of our country. Donate now and help do that.
"We need to hit the ground running. We’ve had an incredible momentum over the last 12 months – we’re the fastest growing party in British Politics, we’ve been pulling off stunning result after stunning result in by-elections across the UK – but this is the real test.
Get Quotes on Home Insurance
"And we’ve got to hit the ground running and if we’re going to change the direction of our country, then we need your help. Please, donate £25 or whatever you can afford today:
"With your help Friend we’re going to throw everything we have at this election; we’re going to be unashamedly liberal and we’re going to stand up for our values. We’re going to elect more Liberal Democrat MPs – and we’re going to change the direction of our country.
Do you think the Lib Dems want you to donate?!
"That’s our goal. And we don’t have any time to waste so please, Friend, donate today and help get our campaign off to a flying start.
Tim Farron MP, Leader of the Liberal Democrats
But other observers were less impressed by the father-of-four's comments.
James Wood posted on Twitter: “The problem is Tim Farron is an idiot.”
And Labour activist Jack Fayter added: "Farron proclaiming that Lib Dems can be any sort of opposition is nonsense."