Speculation has begun over who will replace Johann Lamont after the Scottish Labour Party leader quit with an attack on Westminster colleagues who made her position "untenable".
Ms Lamont accused some in the UK party of treating Scotland as a "branch office" and called for greater autonomy north of the border.
Labour former first minister Lord McConnell said UK party leader Ed Miliband had questions to answer, while his predecessor Henry McLeish said the party faced a problem of ''historic, epic proportions''.
First minister-in-waiting Nicola Sturgeon said the resignation revealed Labour to be in "complete meltdown" in Scotland.
Former prime minister Gordon Brown MP, Jim Murphy MP and interim leader Anas Sarwar MP have all been linked to the leadership role.
Michael Connarty, Labour MP for Linlithgow and East Falkirk, has backed Mr Brown for the job as ''a towering figure'' who was ''speaking the language of the people of Scotland''.
He told BBC Radio Scotland's Good Morning Scotland programme: ''We should be talking about Gordon and Gordon alone. I'll be seeking him out and so will other people.''
Mr Brown issued a short statement that read: "I am sorry to hear that Johann has resigned.
"She brought determination, compassion and a down-to-earth approach to the leadership and deserves great credit for taking on the challenge after 2011.
Mr Murphy had previously ruled himself out of mounting a challenge to Ms Lamont and urged the party to unite around her last weekend.
Mr Sarwar praised the former leader's "selflessness" in standing down, adding: ''The Scottish Labour Party will now consider and set out in due course the process for electing a new leader as we continue to work to return a Labour government at the general election in 2015 and in the Scottish Parliament elections in 2016."
Glasgow MSP Ms Lamont had pledged to lead Labour into the 2016 Holyrood elections despite questions over her political future in the wake of last month's Scottish independence referendum, in which large numbers of Labour supporters voted to leave the UK.
Announcing her resignation, Ms Lamont told the Daily Record: ''Just as the SNP must embrace that devolution is the settled will of the Scottish people, the Labour Party must recognise that the Scottish party has to be autonomous and not just a branch office of a party based in London.
''Scotland has chosen to remain in partnership with our neighbours in the UK. But Scotland is distinct and colleagues must recognise that.
''There is a danger of Scottish politics being between two sets of dinosaurs ... the Nationalists who can't accept they were rejected by the people, and some colleagues at Westminster who think nothing has changed.''
She also attacked those who had attempted to undermine her position as she sought to reform the party in Scotland.
''The Scottish Labour Party should work as equal partners with the UK party, just as Scotland is an equal partner in the United Kingdom. Scotland has chosen home rule - not London rule.''
Ms Lamont said some Westminster colleagues "do not understand the politics they are facing" in Scotland after the referendum.
She was standing down "so that debate our country demands can take place''.
Ms Lamont said she was proud of what had been achieved under her leadership, particularly the No vote in the referendum.
She urged Westminster politicians not to hamper the Smith Commission process on devolving further powers for the Scottish Parliament.
Ms Lamont pledged to back her successor, adding: ''I believe the Scottish Labour Party is a family. It is my family. And I hope it is led by someone who knows how to treat family members properly.''
Responding to her resignation, Miliband thanked Lamont for the "successful No vote in the Scottish referendum campaign."
"Having elected a new leader, I believe the party will show the same will and determination it did in the referendum campaign to help us to victory in the general election of 2015 and the Scottish elections of 2016," he added.
But Lord McConnell told Good Morning Scotland: ''She (Ms Lamont) clearly blames today publicly Ed Miliband and those around him and that's a very serious accusation that requires answers, and it requires answers not just from him but from those closest to him.''
Mr McLeish told BBC Radio 4's Today programme Ms Lamont had been unable to lead the party in Scotland amid ''constant sniping'' from MPs.
He said: "There has been a suffocating atmosphere of control that Westminster have been trying to put on Scotland. That's what led Johann, I think, finally to leave.
''Labour in Westminster, Labour in London has not a clue about the realities of Scottish politics. Johann has been badly advised. The influences on Ed Miliband have not been helpful.
''Now what we've got is a situation after a decade that Labour is still in denial in the UK and ... if there's any hiccup in the number of MPs we send to Westminster in 2015 this could be catastrophic for Ed Miliband's effort to become prime minister.''
Ms Sturgeon highlighted polling analysis putting Labour at 26% for next year's UK general election, compared with SNP support of 43%.
She said: ''Johann Lamont carries my personal best wishes, including in continuing to represent the people of Glasgow Pollok, but there is no question that her shock resignation reveals Labour to be in complete meltdown in Scotland.
''Labour were already a party in crisis, and Johann Lamont's resignation - caused by infighting and deep division - has plunged them to a new low.''
First Minister Alex Salmond said: ''It was always very clear that Johann Lamont was never able to be meaningfully in charge of Labour in Scotland, and that is laid bare in dramatic fashion in her resignation comments.''