David Miliband announced today that he would be standing for the Labour leadership, saying the task for the party was to present itself again as "an alternative government".
The former foreign secretary, surrounded by a handful of supporters on the steps of the Houses of Parliament, said: "We now have a contest to succeed Gordon as leader of the Labour Party.
"I will stand as a candidate. I do so with humility in face of the responsibility this post brings and passion for the causes and values that led me to join our party."
Mr Miliband became the first Labour figure to declare his candidature since Gordon Brown announced last night he was standing down as leader with immediate effect.
Setting out his stall for what will be a heated contest, Mr Miliband said the party had to be ready to return to power.
"We must renew but we must be ready for government," he said.
"We live in a new political world, and the responsibility of office may return sooner than people might think.
"I am standing because I believe I can lead Labour to rebuild itself as the great reforming champion of social and economic change in this country."
He said: "The decision of the Liberal Democrats to join a Conservative Government is a momentous one.
"It creates an enormous responsibility for the Labour Party, revitalised in the right way, to represent all shades of progressive opinion and present itself as an alternative government.
"That is the task in front of us."
Mr Miliband said: "We have achieved a great deal in government. But this is a new era. New dangers, new opportunities, new possibilities.
"No longer the party in government, we must be the movement of real change right throughout the country.
"Deep renewal in our party, deep roots in our country, real engagement with its people."
Mr Miliband will officially launch his campaign in his South Shields constituency next week, but he will begin travelling around the country tomorrow, focusing on those areas which deserted Labour at the General Election.
"As from tomorrow I will be going around constituencies of this country, mainly constituencies that we lost but also some that we won, to talk to people in our own party, but also people who didn't vote for us, to understand why, to understand their hopes, their aspirations and their fears," he said.
In a clear attempt to reach out to disaffected Liberal Democrats, Mr Miliband said Labour could be the "great unifying force of all shades of centre and centre-left opinion".
Before he officially declared his candidacy, Mr Miliband won the backing of former home secretary Alan Johnson, who had been considered a contender for the title.
Mr Miliband is set to face challenges from former Cabinet colleague Ed Balls and possibly his own younger brother, Ed.
The former foreign secretary said he hoped the leadership contest would be "open, warm, generous, comradely".