After a difficult week in Brighton in which questions over Gordon Brown's health and the loss of support from The Sun threatened to overshadow the Prime Minister's attempt to launch a fightback, Ms Harman told delegates: "This country needs a Labour Government. There could not be more at stake."
Warning that the Conservatives were already "measuring up for the curtains in Downing Street", Ms Harman said that David Cameron was arrogantly taking voters for granted and that Labour had to fight to deny him victory in the election expected in the spring.
Effectively firing the starting pistol on a long election campaign which will stretch through the coming months, she said: "We are determined. We are committed. Together we'll fight for those we represent. Together we'll fight for a prosperous and fair Britain.
"And most importantly, we'll fight to win."
As the conference closed with a rendition of the Red Flag, Mr Brown published a campaign document for activists, setting out how they should frame the election as a big choice between sharply divergent visions for the future presented by Labour and the Tories.
"We are fighting for a better future for our country," wrote the Prime Minister in a foreword to The Choice For Britain, which highlights Labour plans to rein in bankers' bonuses, clamp down on anti-social behaviour and restore "sustainable, low-carbon growth".
"The Tories represent change the many cannot afford. That is the stark choice before the British people at the next election.
"By 2015, we want our country to be fairer, greener, more prosperous and democratic. A new Labour Government based on our enduring mission to offer everyone, not just the privileged few, the chance to succeed."
Ms Harman told delegates that the Conservatives would fight the election on a "millionaires' manifesto" of savage cuts to public services and inheritance tax breaks for the wealthy and economic policies which would plunge Britain back into recession. Labour would not "let the Tories off the hook", she said.
"We all know that without Gordon's leadership, with David Cameron's policies, this country would have faced not just a recession - from which we are beginning to recover - but a devastating global depression which would have lasted a decade," she said.
"A Labour Government has given people real help to cope through the recession. Our task as a Government is to make sure that we keep on with that help. Our task as a party is to make sure that we don't let the Tories wreck it."
And she turned one of the Tories' slogans against them, saying: "We mended the roof when the sun shone - the Tory plans would be like a wrecking ball which would demolish the recovery.
"When it comes to public investment and public services, let's be clear about something. This is not just a question of their misjudgment. It's not a question of competing sets of figures. This is a question of what you believe in.
"The Tories see public investment as a millstone round the country's neck and a drain on the public purse - we see public services as a vital part of what makes Britain prosperous and fair."
After yesterday warning The Sun not to try to "bully" Labour, Ms Harman was targeted with a cheeky jibe from the tabloid, which featured a topless Page 3 girl called "Harriet, 20, from Peckham". The equalities minister laughed off the stunt, and hit back by telling delegates: "The press does not decide who will govern this country. The people do."
However it was clear that ministers and activists were still smarting from about paper's devastatingly-time announcement, on the evening after Mr Brown's make-or-break speech, that it was switching its allegiance to Mr Cameron's Conservatives. Foreign Secretary David Miliband won applause by saying: "The Earth does revolve around the sun, but not the one printed in Wapping."
Ms Harman hailed Mr Brown's speech as "the highlight of the week" and also paid tribute to Lord Mandelson, who became the darling of the conference as he said: "If I can come back, we can come back."
"Tony Blair said that New Labour will have come of age when Labour loves Peter," said Ms Harman. "Gordon talked about his special relationship with Peter. And as for me and Peter, I think it's taken us both by surprise. Things are moving so fast I'm even thinking of taking him home to meet my mum."
Mr Miliband denied Labour had "run out of steam" and rounded on defeatists within its ranks, telling delegates: "When members of this party, even MPs, say that... we could use a spell in Opposition, tell them: don't do the Tories' dirty work for them."
Mr Brown insisted that Labour could still "turn it round" to win a historic fourth term in power.
Asked why voters should vote him back into power, the PM told the BBC: "Because we are going to create a fairer, more responsible, more accountable Britain, where people feel that they have got a stake in this country and they know they have got the Government on their side and you have a Prime Minister that is always working - every day, every morning, every evening - for the hard-working majority of people in this country."
Elsewhere in the conference, Defence Secretary Bob Ainsworth indicated that a decision on more troops for Afghanistan would be taken within months, but said he would not commit extra British forces unless they could be adequately supplied with kit and resources.
And International Development Secretary Douglas Alexander said the Government was "ready to act" to help those affected by the tsunami and earthquakes in the Pacific Ocean.
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