The former foreign secretary, David Milliband, 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.'
Announcing his candidacy outside the House of Commons, Mr Miliband paid tribute to Gordon Brown as a "towering figure" and wished the new Conservative-Liberal Democrat government well.
He said he was standing because he believed he could help Labour "rebuild" itself as a force for "social and economic change" in the country while in opposition.
"We have achieved a great deal in government but this is a new era with new dangers, opportunities and possibilities," he told journalists.