The Prime Minister declared she would "unashamedly" campaign for "future of the United Kingdom" in the wake of her bombshell statement.
Mrs May's decision to go to the polls on June 8 sparked a furious response from Nicola Sturgeon who branded it a "huge political miscalculation".
The First Minister also said she would use the ballot to bolster her plans for another referendum on breaking up Britain.
She wants an independence vote as early as autumn 2019 despite Mrs May rejecting the call.
The issue was already expected to dominate next month's council elections.
Scottish Tory leader Ruth Davidson today said she aimed to “send a strong message that we oppose the SNP’s divisive plan for a second referendum” in the national contest.
Theresa May vowed to battle for the Union after her snap general election announcement
Earlier, Mrs May left Westminster reeling as she asked for voters to give her their trust to lead Britain out of the EU.
It will once again give people the opportunity to reject the Tories’ narrow, divisive agenda
MPs are set for Commons vote on Wednesday to rubber-stamp her shock decision.
Both Mrs May and Number 10 have repeatedly insisted she would not seek a general election before the scheduled 2020 poll.
But today she argued she needed a fresh mandate from the electorate to strengthen her hand in the forthcoming Brexit negotiations.
Get Quotes on Home Insurance
She declared she would 'unashamedly' campaign for the 'future of the United Kingdom'
She accused Labour, the Lib Dems, the SNP and Lords of "game playing" and warned this could undermine talks with Brussels.
The SNP has threatened to block flagship Brexit legislation, the Great Repeal Bill, in a row over the repatriation of powers while also pursuing a second independence referendum.
Mrs May told ITV News: "I will be out there championing the cause of a United Kingdom.
"I believe that we are stronger as a United Kingdom. I think this Union we have between Northern Ireland, Scotland, England and Wales is a very precious Union.
"I think we all benefit by it. And I will unashamedly be out there campaigning for the future of the United Kingdom."
Westminster was plunged into turmoil this morning when Downing Street announced that Mrs May was about to address the nation.
She emerged from Number 10 shortly after 11am to make her electrifying statement.
Speaking to ranks of television cameras in a windswept Downing Street she said "the only way to guarantee certainty and security for years ahead is to hold this election".
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
"We need a general election and we need one now," she said.
"At this moment of enormous national significance there should be unity here in Westminster, but instead there is division.
"The country is coming together, but Westminster is not.
With an opinion poll today giving the Tories a mammoth 21-point lead, the Prime Minister was on course for a landslide election victory that could deliver a majority of up to 200.
Ms Sturgeon said she would fight the contest arguing that a vote for the SNP is needed to stand up against Conservative plans for a "hard Brexit" and continuing austerity.
“It will once again give people the opportunity to reject the Tories’ narrow, divisive agenda as well as reinforcing the democratic mandate which already exists for giving the people of Scotland a choice on their future,” Ms Sturgeon added.
Nicola Sturgeon branded the deception to go go the polls a 'huge political miscalculation'
The SNP leader also claimed claimed Mrs May had made a “huge political miscalculation” describing her bid as “one of the most extraordinary U-turns in recent political history.”
Ms Sturgeon said Mrs May was “clearly betting” that the Tories can win a bigger majority in England given the “utter disarray” in the Labour Party.
She added: "I've got a mandate for a second referendum; I won that mandate at the Scottish Parliament elections last year and of course the Scottish Parliament has since voted by majority for that position, so that mandate is there and it is clear.
"This election will be about the kind of country we want Scotland to be and whether we want the Tories to have a free hand in determining that or whether we want to make sure that we stand up for Scotland's public services, for public spending, against further Tory austerity."
Her spokeswoman later said the SNP had been clear since Holyrood was established that "the mandate for a referendum comes from the Scottish Parliamentary elections".
She added: "Undoubtedly this will be an issue in the referendum, but the position is one that has been set out.
She also said she would use the ballot to bolster her plans for another independence referendum
"I think the Prime Minister might have miscalculated if she thinks that having this is going to give her a clear mandate in Scotland against a referendum."
Ms Sturgeon has yet to set out the "next steps" in her plans for an independence vote but the spokeswoman said today's announcement "may affect the timing in a short term sense."
Meanwhile, Tory leader Ms Davidson was confident of her party’s chances of increasing its tally of only one MP north of the Border.
She said: “We know the SNP will use this campaign to try and manufacture a case for separation.
"And with Jeremy Corbyn having already said he is ‘absolutely fine’ with an immediate referendum, we also know that Labour can’t be relied to stand up to them.
"By contrast, the Scottish Conservatives have the strength right across Scotland to stand up for people who oppose the SNP’s plans.”
Scottish Labour leader Kezia Dugdale said: "At this election, the choice again will be clear: a Tory Party intent on a hard and damaging Brexit; or a Labour Party that will oppose a second independence referendum and fight for a better future for everybody."
She added: "We will work tirelessly to elect Jeremy Corbyn Prime Minister and deliver a Labour government."
Willie Rennie, the Scottish Liberal Democrat leader, said: "At this election we will stand proud for a United Kingdom within the European single market.
"We will avoid a disastrous hard Brexit and keep Britain united.
"Liberal Democrats are only party that can stop a Conservative majority across the UK.”
Labour and the Lib Dems signalled that their MPs will back a Commons motion on Wednesday to trigger the June 8 poll, virtually guaranteeing Mrs May the two-thirds Commons majority needed to pass the legislation. The SNP has also indicated its 54 MPs will not attempt to block it.
Mrs May informed the Queen about her decision on Bank Holiday Monday and then telephoned Ms Davidson before announcing the move to her stunned Cabinet today.
The Prime Minister made up her mind about the dramatic move during her walking holiday with her husband Philip in North Wales over the Easter break.