Praising MPs for backing her during a stump speech in the north-west of England, the PM said: "It's great to be here in Bolton, fresh from the House of Commons, fresh from winning a vote in the House of Commons, which has approved my decision to hold a general election on June 8".
For months, May had resisted calls from within her own party to push for another election before the current parliament was due to end in 2020. "I believe in campaigns where politicians actually get out and about and meet with voters".
Jeremy Corbyn accused the Conservatives of presiding over falling incomes, a struggling health service, cuts to school funding and rising child poverty, and asked: "If the Prime Minister is so proud of her record, why will she not debate it?"
Parliament will be dissolved on May 2.
All sides are now gearing up for the fourth major election in four years, after last June's shock referendum vote for Brexit, the 2015 general election, and the 2014 Scottish independence vote.
Despite Corbyn's bravado, his party is demoralized and divided under his left-wing leadership and is expected to fare badly.
Angus Robertson, leader of the SNP in Westminster, said May had changed her mind because she now realised the Brexit discussions were going to be much harder than Eurosceptics had imagined.
The government's current working majority is only 17 seats.More news: Aitzaz Ahsan expects no breakthrough in Panama papers verdict
"She expects a coronation and not a contest", Farron said, urging voters to back his strongly pro-EU party to stop a Conservative landslide. The Lib Dems now have just nine seats in Parliament. Former Conservative finance minister George Osborne, a powerful voice in favour of Britain's European Union membership during the referendum campaign, also said he would not seek re-election.
May, who had previously denied she would hold an election before 2020, said she had "concluded the only way to guarantee certainty and security for years ahead is to hold this election".
"I want this country to be able to play the strongest hand possible in those negotiations to get the best possible deal because that's in our long-term interests".
"It was with reluctance that I decided the country needs this election, but it is with strong conviction that I say it is necessary to secure the strong and stable leadership the country needs to see us through Brexit and beyond", she said. The leaders of the 27 remaining European Union countries will meet on April 29 to agree the bloc's negotiating position, which will then be translated into a legal text by the European Union commission in May.
According to opinion polls, May is on course to win a crushing majority that would strengthen her position politically as she goes into hard negotiations with Brussels about Britain's withdrawal from the EU. But the Prime Minister has refused to take part in televised leaders' debates. "I will be asking for the British people for a mandate to complete Brexit and make a success of it".
May has also ruled out taking part in TV debates with Corbyn and her other political opponents, saying she believed elections were all about "getting out and about and meeting voters" and, in her words, "knocking on doors".
There were passionate exchanges during Prime Minister's Question Time.
And Plaid Cymru leader Leanne Wood - whose profile received a major boost from her involvement in two of the 2015 broadcasts - said: "Theresa May should be empty chaired if she doesn't show up to any planned TV debates".