He said in a statement: "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."
The Lib Dems have consistently called for a second referendum on the final Brexit deal Ms 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.
The Lib Dems later tweeted to say they had gained 1,000 new members in the hour after Ms May's annoucement.
Ms May announced a snap general election for 8 June, saying: "We need a general election and we need one now."
She said a strong government was needed before pushing ahead with Brexit talks with the European Union.
Stephen Crabb, who ran against Theresa May for leadership of the Conservative Party last year, tweeted his enthusiasm for the news, saying: "Sun is shining. Perfect moment for an election. Let's go."
Meanwhile, Labour MP for Middlesbrough South and East Cleveland Tom Blenkinsop announced he would not be standing for re-election.
He tweeted a link to what is believed to be a statement on his personal website, but the site has crashed due to heavy traffic.
Former prime minister David Cameron called Theresa May's decision to hold a snap general election "brave and right" in a tweet.
"My very best wishes to all Conservative candidates," he added.
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