Ed Miliband has launched Labour's biggest voter registration drive in a generation in an attempt to rebuild the support which delivered its landslide general election victory of 1997.
Following last week's local elections - which saw the lowest turnout in a decade - the Labour leader said the party needed to regain the trust of an electorate which has lost confidence in politicians.
With an estimated six million people already missing from the electoral register, he said the party had "a very long way to go to build the deep trust we need, to build the allegiance we need, to build the enthusiasm we need".
Addressing the Progress annual conference in London, he said Labour must show it was prepared to stand up for the interests of the many in the face of a Tory-led Government representing only the powerful and privileged.
"We now have an opportunity and we must seize this moment," he said. "I want the British people to understand how the Labour Party is changing. To understand the character of our party, how it can reach out. Our work to make that happen is well under way but it now must intensify."
Mr Miliband also mounted a fresh attack on the Government in the wake of the latest revelations at the Leveson inquiry, accusing it of having "bent over backwards" to help Rupert Murdoch's media empire while Britain was sliding into a double-dip recession.
"The reason they have lost their way so badly in the last few weeks is not some accident, not some set of ministerial mess-ups. It is much deeper than that," he said. "It is about the revelation of who they really are, the character of the Government, who they really stand for."
He acknowledged, however, that Labour in office had been too slow to take on powerful vested interests like the banks, the utility companies and the big media giants.
"Let's be frank about this - the British public lost faith in who we stood for. We became 'one of them' rather than 'one of us', and we must put that right."
If Labour was to achieve the goal of securing the biggest general election turnout since Tony Blair's victory in 1997, he said it would have to show it had truly changed as a party.