"After spending the last 20 years of my life leading humanitarian and reconstruction projects overseas, I came home to New Zealand because I'm passionate about this country.
"We have a history to be proud of, but I believe our best years lie in front of us. But to really take this country forward, we need a change of government. We need a progressive government with fresh ideas.''
Labour Party deputy leader Grant Robertson said he had not ruled out, or in, a leadership bid.
Mr Robertson said he had only found out about Mr Shearer's decision to resign mid-morning, and he needed time to reflect.
"I am going to take some time ... to talk to my caucus colleagues, to talk to party members and then I'll make a decision.
"... I'll come back to you when I've made a decision.''
Speaking on Maori TV's Te Kaea tonight, MP Shane Jones would neither confirm nor deny whether he would stand for Labour's top spot.
When asked whether it was time for him to lead the party, Mr Jones said in Te Reo: "Only time will tell.''
He also said he would have to speak to his colleagues before deciding whether to launch a leadership bid.
"I haven't yet approached them.''
Labour MP David Cunliffe said today he would decide over the next few days whether to have a further go at getting the Labour leadership.
On the way into Parliament minutes after Mr Shearer's announcement today, Mr Cunliffe said he had "made no decisions'' about his own plans to run for the job and had learned of Mr Shearer's decision only at the caucus meeting called just before it was publicly announced. Mr Cunliffe lost to Mr Shearer in the runoff for the job after the 2011 election.
"I've made no decisions. I'll be consulting with party members, my LEC [electorate committee], supporters and I'll make up my mind over the next few days.''
Andrew Little, another MP regarded as having leadership aspirations, also refused to rule out trying for the role but said he had given "no thought'' to the matter. "These are long, drawn out affairs sometimes.''
Mr Shearer was a man of ``honour and integrity``in a difficult job, Mr Little said. ``It's really hard for us all that it should end in this way.''
Labour's finance spokesman David Parker, who had withdrawn from the leadership race in 2011 to boost Mr Shearer's chances against Mr Cunliffe, said he would not put his name forward again.
Former leader Phil Goff also ruled out another tilt.
Labour Whip Chris Hipkins, who was at Mr Shearer's side at the press conference, tweeted shortly afterwards: "It has been an absolute privilege to work with and get to know David Shearer. He is a man of great integrity, honesty and compassion.''
"I respect David's decision to stand down. David has put our collective ambitions for NZ ahead of his own. He has humility and dignity.''
New Zealand First Leader Winston Peters said Mr Shearer was a "really decent guy and it was always going to be tough to be the leader during the recovery period for any political party''.
"I think that it's time for some people to reflect on where they want to go because a few months from now the political scene will change and I think will change dramatically and those parties that are in opposition need to be aware of that.''
Maori Party Co-leader Te Ururoa Flavell said he was shocked by Mr Shearer's decision but added it was probably inevitable.
"Today's not necessarily about making a statement about him or what he has or hasn't done, I just feel sorry for the guy.
"He did his best, and that's all you can ever ask for a person in here.''
Political commentator Dr Bryce Edwards said David Cunliffe and Grant Robertson were likely to be the two front-runners for the party leadership.
"In that case, I think, Cunliffe probably has the edge, He's the assured performer and he's been a minister before and has a fair degree of public profile and respect already.
"The main thing holding Cunliffe back, of course, is that he's not particularly well liked by his colleagues.''
Green Party Co-leader Metiria Turei said she was surprised Mr Shearer stepped down, but said it was brave he resigned rather than be removed.
"I think he did a good job over the last few years.''
The Mana Party's Hone Harawira said he always found Mr Shearer to be "very very friendly and very open''.
"I think that was probably his downfall - nice guys don't last long in this game.''
Act Leader John Banks said Mr Shearer was "knocked down'' by Prime Minister John Key in Parliament on Tuesday when the Labour leader produced two dead snapper in the House to use as props in his attack on proposed cuts to recreational fishing quota.
"Floored, king hit, knocked down and he was never going to get up.''
Mr Banks said Mr Shearer's leadership of Labour was "useless, the worst leadership of the Labour Party since Bill Rowling, and I was here in Parliament with Bill Rowling''.
However, politics aside, Mr Banks said Mr Shearer was "a very good man''.
"He's got two Russian adopted children - he's a very good man.''
***
Timeline:
10 July 2013: Denies a leadership coup is brewing, after rumours MPs had drafted a letter of no-confidence
November 2012: Leadership endorsed by a caucus vote, held to end speculation of a leadership crisis
December 2011: Wins Labour leadership race after Phil Goff's resignation in November
2011: Retains his Mt Albert seat in general election
2009: Enters parliament after winning byelection in Mt Albert, Helen Clark's former seat
2000-2002: Adviser to then Foreign Minister Phil Goff
