"It is a recent and reluctant decision," May said in an urgent statement that followed a routine Cabinet meeting. "I will move a motion in the parliament tomorrow."
Underlining the opposition of the other parties, and their attempts to block the Brexit decision, May complained about a lack of unity in parliament.
"At this moment of enormous national significance there should be unity here in Westminster, but instead there is division," she said.
May recalled that in recent weeks, the Labour Party had threatened to vote against the deal reached with the European Union, while the Liberal Democrats "have said they want to grind the business of government to a standstill".
"The Scottish National Party say they will vote against the legislation that formally repeals Britain’s membership of the European Union.
"And unelected members of the House of Lords have vowed to fight us every step of the way," she continued.
May said the opponents were "wrong" to believe that the government’s majority is small and "they can force us to change course".
"They underestimate our determination to get the job done and I am not prepared to let them endanger the security of millions of working people across the country," she said.
"The government has a plan for Brexit that will allow the U.K. to regain control of its laws and borders. ... This is the right approach."
To hold a snap election, May needs a two-thirds majority vote in parliament on a motion she said would be submitted to the House of Commons on Wednesday. If the motion is passed, parliament will dissolve in May.
Early reactions from political parties signaled that the motion will receive the necessary 434 votes in parliament.
“I welcome the prime minister’s decision to give the British people the chance to vote for a government that will put the interests of the majority first,” Labour Party leader Jeremy Corbyn said of the decision.
“This election is your chance to change the direction of our country,” pro-EU Liberal Democrats leader Tim Farron said of the announcement.
“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,” he said.
Scotland’s First Minister Nicola Sturgeon called the decision a “huge political miscalculation” by May “in terms of Scotland.”
“This announcement is one of the most extraordinary U-turns in recent political history, and it shows that Theresa May is once again putting the interests of her party ahead of those of the country,” she said.
After Brexit Sturgeon has pushed for a new referendum on Scottish independence, a move opposed by May.
A spokesman for the prime minister said May spoke to the queen by phone on Monday before making the announcement.
May has opposed the idea of a snap election since she came to office following the resignation of her predecessor David Cameron.
Recent opinion polls suggest a clear lead for May’s Conservative Party.
British voters decided to leave the European Union in a referendum last year, signaling the end of the country's 44-year-long membership in the bloc.