He was speaking as senior Labour politicians claimed the embattled party could still win the next general election.
Mr Miliband and deputy Labour leader Harriet Harman used their speeches on the closing day of conference to try and resurrect party morale, after the Sun switched its allegiance to the Tories after 12 years.
Attendance rates at this year's conference in Brighton have been poor for a pre-election rally. And although predictions of leadership wrangling failed to materialise, many activists privately admit that the atmosphere has been lacking. Mr Miliband – who two years ago almost launched his own challenge to Gordon Brown for the leadership – warned his party not to surrender without a fight. And he rounded on critics within the party and insisted Labour still had the energy and ideas to win the next election.
The party was still made up of "agents of change", he said, a remark that will be interpreted as a coded reference to Mr Miliband's desire for change at the top of Labour.
He said: "When members of this party, even MPs, say 'Nothing much has changed, we could use a spell in Opposition', tell them 'Don't do the Tories' dirty work for them'."
As delegates shouted: "Shame on them!", he added: "If we do not defend the record, no one will."
Mr Miliband also launched Labour's most furious assault on the Conservatives to date, accusing Tory leader David Cameron of showing the "white flag of surrender to euro-extremism" in his own party.
The Tories were "a bunch of schoolboys" and "a national embarrassment", he said.
The Foreign Secretary said the fact Tory MEPs were in alliance with a party that celebrated the Latvian Waffen SS "made him sick".
And he claimed Conservative Party chairman Eric Pickles had refused to condemn the Tories' allies in Europe – the For Fatherland and Freedom Party "because they were only following orders".
"It makes me sick. And you know what makes me sicker? No-one in the Tory party batted an eyelid."
He added: "It's tempting to laugh at the Tory policy on Europe. But I don't want people laughing at my country because a bunch of schoolboys have taken over the government.
"The Tories are not a government in waiting. They are a national embarrassment.
"David Cameron has shown not leadership but pandering, not judgment but dogma, not patriotic defence of national interest but the white flag of surrender to euro-extremists in his party."
The remarks were met with a furious response from shadow foreign secretary William Hague, who demanded Mr Miliband withdraw his comments about Mr Pickles.
Mr Miliband's speech received a standing ovation before Ms Harman – another leadership hopeful – took to the stage to make the closing speech.
She called on activists to "fight to win" and told them: "This country needs a Labour government. There could not be more at stake."
Earlier in the day, the Labour leadership suffered a defeat, paving the way for a return to more left-wing policies after the next general election.
A move by grassroots Labour members to gain more control over the party's policy-making body – the National Executive Committee – was successful, with nearly 67 per cent voting in favour of the move.