In an announcement on Tuesday, May said she was seeking a stronger mandate in talks over leaving the EU.
In an unexpected statement at Downing Street, May said she the election would be held on 8 June 8 - less than halfway through the government's five-year term.
May, who commands only a slim majority in the UK House of Commons, said that a new mandate would strengthen her hand in negotiations in Brexit talks.
She triggered article 50 last month, setting in motion the UK's EU exit.
Her decision is a reversal of policy - a month ago, her spokesperson said there would not be an early election.
Ashley Fox, leader of Conservatives in the European Parliament, told this website, "It is vital that the UK enters the forthcoming talks with the European Union united behind the Prime Minister's negotiating stance.
"A clear victory for the Conservatives in a general election on 8 June will provide the government with a strong mandate to secure the best deal for Britain and develop a new, deep and positive relationship with our European partners.
"It will provide clarity, end uncertainty and help silence those who seek to defy the will of the British people."
He added, "Theresa May's announcement today is a strong, decisive and important step along the road to delivering a Brexit that works for both the UK and the EU27."
UK Greens MEP Moly Scott Cato said, "The Green party has been calling for a general election since June last year because the Prime Minister has no mandate to throw us off a Brexit cliff edge. We can create another future for Britain. I will resist an extreme Brexit."
She added, "The Green party stands to make history in Bristol on 8 June, where the people of Bristol west will have the chance to elect their first Green MP. I've served the area as MEP since 2014 and as an MP I'll be 100 per cent committed to protecting the environment and fighting climate change.
"I represent a party untainted by the unpopular policy of privatisation and I'm determined to give the people of Bristol a vote on the final deal. Bristol is the country's greenest city and I look forward to making history in June."
British MPs also gave their response with the Lib Dem leader, Tim Farron, saying, "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."
SNP leader Nicola Sturgeon commented, "The Tories see a chance to move the UK to the right, force through a hard Brexit and impose deeper cuts. Let's stand up for Scotland."
Elsewhere, former UK Europe Minister Denis Macshane told this website, "This is a cynical Erdoğan type election which will divide Britain and poison the EU negotiations on Brexit.
"May is in complete control today of the political agenda and although she is certain to gain many seats at Labour's expense she will re-enter Downing Street on 9 June without a solution to what future for Brexit Britain. On the contrary she is likely to harden her line on Brexit to win English nationalist votes."
He added, "Scotland and Northern Ireland will not vote Conservative or endorse Brexit so by mid-summer the UK will be more disunited than ever and the EU will see a British prime minister determined to do as much damage to Europe as possible purely for internal party management reasons."