Tony Woodley rips up a copy of The Sun with "Labour's Lost it" front page
Harriet Harman has said Labour "won't be bullied" after the Sun said it would not back Labour at the next election.
Labour's deputy leader said her party was "angry" at the paper's decision, but urged members to use the move to inspire them to victory.
Gordon Brown has shrugged off the Sun's decision by insisting "it is people that decide elections" not newspapers.
Tony Woodley, general secretary of the Unite union, later tore up a copy of the Sun during his conference speech.
Referring to the paper's foreign owner Rupert Murdoch, he told delegates: "We don't need any Australian-American coming to our country, with a paper that's never supported one progressive policy from our party, including the minimum wage, telling us how our politics should be run.
"In Liverpool, we learned a long time ago what to do [with the Sun]," he said, ripping up the paper: "I suggest the rest of the country should do exactly the same thing."
Conservative leader David Cameron has said he is "delighted" at the development as it showed more people recognised his party was "setting the agenda" in the country.
He said that while voters decided election results not newspapers, the Sun was "an important newspaper, with millions of readers" and he wanted to build "the widest and broadest possible coalition for change".
Opening a debate about equalities at the Labour conference, Harriet Harman said she was speaking about "something the Sun knows absolutely nothing about - equality".
She went on: "Let's face it, the nearest their political analysis gets to women's rights is Page 3's news in briefs."
We would would like the support of every newspaper... but it is people that decide elections
She added: "I say to you don't get bitter, get better. Don't get outraged, get out there. Don't get mad, get mobilised.
"We may be the underdog but we won't be bullied... this underdog is biting back," she told the party's conference."
Although it has said it will not support Labour, the Sun has not explicitly endorsed the Conservatives, saying they have to do more to earn voters' trust.
The BBC's political editor Nick Robinson said Labour had known the decision was coming for some time and was designed, coming immediately after Gordon Brown made his last conference speech before the election, to have maximum political impact.
The Sun claimed that it helped win the election for John Major in 1992 but senior Labour figures like John Prescott have tried to play down the significance of its decision this time around.
Although the Sun still sells more than three million copies a day, they argue its political influence has waned and people get their information from a much wider range of sources than in the past.
Harriet Harman attacks the Sun
Former Labour deputy leader Margaret Beckett said the Sun's switch was a "problem" for the party but not "insurmountable".
Mr Brown, whose predecessor Tony Blair did much to woo The Sun in the 1990s, said he did not "get out of bed" each morning thinking what the newspapers were saying about him.
"In the end we would like the support of every newspaper, you'd like to have the support of lots of people that are not giving you support but it is people that decide elections," he told the Today programme.
He said he believed Sun readers would back new policies on tackling anti-social behaviour, and cancer test guarantees would appeal to the "mainstream middle" of British society.
'No blank cheque'
The Sun's associate editor Trevor Kavanagh said the paper had been debating whether to withdraw its support from Labour for some time.
Although the paper's proprietor Mr Murdoch had been "involved" in the decision, he said it was the paper's readers who had really driven the switch.
"They have turned off Gordon Brown and the Labour Party," he told the BBC's Daily Politics.
However, he said the Conservatives must say more about their policies on the economy, immigration and Europe.
"This is no blank cheque," he said. "We want them [the Tories] to put more flesh on the bones."
Former home secretary David Blunkett said Labour still needed to get its message across to hostile newspapers or it risked returning to the "mud-throwing" of the 1980s.
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