Announcing himself as a candidate, David Miliband said the task for the party would be to present itself again as “an alternative government”.
Mr Miliband, who served as foreign secretary in the Brown administration, 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.”
“I will stand as a candidate. I do so with humility in face of the responsibility this post brings.”David Miliband
He committed to touring constituencies where Labour lost seats during his leadership bid to talk those who did not vote for the party as well as supporters.
Mr Miliband has emerged as an early frontrunner in the contest, and was frequently regarded as a possible challenger to Mr Brown in the event of a forced leadership contest.
As he made his address at parliament’s St Stephen’s entrance, Mr Miliband was flanked by 15 MPs.
Among them, several former ministers including Jim Fitzpatrick, David Hanson and Phil Woolas.
There were also newly-elected Labour MPs Pamela Nash, Julie Elliot, Anas Sarwar and Valerie Vaz.
He is also backed by former home secretary Alan Johnson, who was also seen as a possible candidate.