Labour leader Jeremy Corbyn has vowed to "overturn the rigged system" by putting power and wealth back in the hands of "the people".
In his first major general election speech, he said 8 June's poll was not a "foregone conclusion" and Labour could defy the "Establishment experts".
He also said Labour would not back a second EU referendum.
Theresa May said the election was about ensuring "strong and stable leadership" for the UK.
It was also about strengthening the government's Brexit negotiating hand, she said.
The PM is hoping to convert the Tories' double digit poll lead into a bigger Commons majority.
Her decision to hold a general election - after previously insisting she would wait until 2020 - took her rivals and many in her own party by surprise.
Mr Corbyn could have blocked it in Parliament but instead ordered his MPs to back the snap poll in a Commons vote on Wednesday.
After his speech, the Labour leader was asked to rule out backing a second EU referendum - replying that he respected the result but that the UK had to have continued access to the EU single market and should not "tear up the workers' rights agenda, the environmental protection agenda, or any human rights agenda".
Shadow chancellor John McDonnell also declined to rule one out, telling the BBC the government should "put the deal to Parliament and possibly to the country overall".
But asked later, Mr Corbyn's spokesman said: "A second referendum is not our policy and it won't be in our manifesto."
The Conservatives said it was "yet more evidence" of "Labour chaos".
The Labour leader looks set to run an anti-establishment campaign, presenting himself as a champion of the powerless against political and business elites.
He attacked the "morally bankrupt" Conservatives who he said would not stand up to tax avoiders and other members of a "gilded elite," who were extracting wealth "from the pockets of ordinary working people".
Labour would "end this racket" and "overturn the rigged system," he told an audience of Labour supporters in London.
He also said Labour was the only party that would "focus on the kind of country we want to have after Brexit" - dismissing Mrs May's election campaign as an "ego trip about her own failing leadership".
And he insisted all of Labour's policies, including an increase in corporation tax for big business and more money for carers and a £10 an hour minimum wage, were fully costed.
Addressing Labour's poor opinion poll ratings, he said he was given a 200/1 chance of becoming Labour leader in 2015 and he defied those odds.
Assessing Corbyn's speech: By Iain Watson
Jeremy Corbyn provided two very big clues today to how he will fight the campaign - and for two main reasons.
First, he is opting to have the clearest dividing lines between government and opposition in more than three decades - ever since Labour pledged to scrap nuclear weapons and leave the EU in 1983.
He suggested the Conservatives were "morally bankrupt" and attacked the elites.
But he is also drawing a clear line between his leadership and his party's New Labour past.
He didn't just attack the "fat cats" and the "tax dodgers" - he denounced the "wealth extractors" of big business and the City - and took a pop at the media while he was at it.
What was interesting was that uncensored, unvarnished Corbyn was more passionate and fluent than in many of his much-criticised performances in the Commons.
Mr Corbyn said: "Much of the media and Establishment are saying this election is a foregone conclusion.
"They think there are rules in politics, which if you don't follow by doffing your cap to powerful people, accepting that things can't really change, then you can't win.
"But of course those people don't want us to win. Because when we win, it's the people, not the powerful, who win."
He added: "They say I don't play by the rules - their rules. We can't win, they say, because we don't play their game.
"They're quite right I don't. And a Labour government elected on 8 June won't play by their rules."
He added that those rules "have created a cosy cartel which rigs the system in favour of a few powerful and wealthy individuals and corporations".
Mr Corbyn said: "It's a rigged system set up by the wealth extractors for the wealth extractors."
The Labour leader singled out tycoon Sir Philip Green, who faced heavy criticism over the BHS pensions saga, and Sports Direct boss Mike Ashley in his speech, saying they should be "worried about a Labour government".