Former first minister Henry McLeish said Labour faced a problem of "historic, epic proportions".

"I think Johann is absolutely right to make the comments she has made," he told BBC Radio 4's Today programme.

"For a decade now the party have been in decline and the SNP have been in the ascendancy. There has been a failure to rise to the devolution challenge.

"Overall though 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."

Mr McLeish said: "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."

Mr McLeish said Ms Lamont had been unable to lead the party in Scotland amid "constant sniping" from MPs.

"It's interesting to know that while we've had devolution of government from Westminster over the last 20 years, we've had no devolution of political power from the Labour Party," he added.

Mr McLeish said he had yet to decide whether to support any candidate in the forthcoming leadership contest.

"My main concern is that anyone taking the job now has got to realise that the political plates have shifted between the UK and Scotland," he said.

Lord McConnell told BBC Radio Scotland's Good Morning Scotland programme he was "very, very angry" and said Mr Miliband had questions to answer.

He said: "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."

Lord McConnell added: "I have had my concerns for some time about the way in which the Scottish Labour Party was struggling to set out a vision for the 21st century and a positive vision that would take us from opposition back into power.

"I think the responsibility for that difficulty is shared amongst a number of different people and I'm absolutely certain that Johann Lamont would take some responsibility herself for that.

"But to undermine her position in this way, when the position of the Scottish Labour Party leader was clarified so clearly three years ago, is in my view entirely the wrong way to go, and I think there are serious questions that need to be asked about what has happened in these past two weeks, but also that this issue has to be resolved in advance of the election of the next leader of the Scottish Labour Party."

He added: "I think the party in Scotland should rally round her and ensure that the situation is resolved and that the leader of the party in the UK accepts that situation and makes it very clear that from now on he will not act in this way again, and then we can move forward to elect a successor."

Interim leader Anas Sarwar MP said: "Johann Lamont's decision to put the interests of the Labour Party and the Labour movement ahead of her own shows once again her selflessness.

"When the Scottish Labour Party needed someone to step up, the person who answered the call was Johann Lamont. Johann lifted the party when it needed it most, led the party to success in the 2012 council elections, to by-election wins and, most importantly, to a successful referendum result which saw over two million people vote to remain part of the UK.

"I want to thank Johann for all the support and advice she has given me over the past three years. I am clear she will continue to pursue with great vigour the causes close to her heart, causes which brought her into politics in the first place.

"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.

"In the interim we will also not lose sight of the fact that under the Tories and the SNP inequality is rising, the number of children in poverty is going up, Scotland's NHS is in crisis and college places are being cut and we will continue to hold both Governments to account."

Michael Connarty, Labour MP for Linlithgow and East Falkirk, backed Gordon Brown for the leadership, saying the former PM was "a towering figure" who was "speaking the language of the people of Scotland".

He told Good Morning Scotland: "People are talking about Gordon Brown as leader. I think he should lead us into this next election.

"I think that Gordon has shown he is a Scottish voice, he is a voice for Scotland.

"We should be talking about Gordon and Gordon alone. I'll be seeking him out and so will other people."

The SNP added more reaction this morning. Alex Salmond said: "I always found Johann Lamont to be a spirited opponent in the Scottish Parliament, strongly dedicated to her party, and I wish her well for the future.

"The fact that Scottish Labour are now going to have their fifth leader since the SNP took office in 2007 indicates that their problems are not about personalities - they are much more deep-seated than that.

"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."

His successor, Nicola Sturgeon, 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.

"The scale of the infighting between Scottish Labour and Labour at Westminster is exposed for all to see.

"The London-based leadership pulling the party's strings in Scotland, and Labour campaigning side-by-side with the Tories in the referendum, has proved a deeply corrosive combination which is causing Labour support to plummet in Scotland."

Ms Sturgeon highlighted polling analysis putting Labour at 26% for next year's UK general election, compared with SNP support of 43%.

She added: "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."

Gordon 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.

"I wish her well in the future."

Pat Rafferty, Scottish secretary of the Unite union, said: "Johann Lamont's departure must now spark the desperately needed process of renewal of Scottish Labour's relationship with the people of this nation.

"Unite urges the party to take full and honest consideration of the recognisable concerns raised by Ms Lamont.

"Labour in Scotland must understand that there has been a seismic change in the political landscape in our country. People want a party that grasps this and has the vision to work with them to build the fairer nation they desire.

"The problems facing Labour in Scotland did not start and end with the referendum; the party has not won an election in Scotland since 2007, and it was routed in the Scottish Parliamentary elections of 2011. This ... has been the settled will of the people; a repeated rejection of Labour's offer as inadequate.

"The reality is that voters struggle to tell you what Labour stands for and, where there is a view, it is that the party's timidity in policy is designed to appeal to the more advantaged in our nation.

"The next Scottish leader must forge a clear, social democratic and Scottish identity - tackling head-on the list of inequalities in our economy and society, and delivering the devolved nation our people seek, or it will not find support among the people.

"There can be no more business as usual because politics across these islands are in flux. Labour nationally must understand this, allowing the party in Scotland to divine its destiny, not have one foist upon it from SW1. This moment for genuine change must not be extinguished.

"Unite, as the biggest affiliate to the Labour Party, will be ensuring our members participate fully in this election and that their views are given full voice as we choose Labour's new Scottish leader."

Unite has 165,000 members in Scotland.

In her resignation letter to Jamie Glackin, chair of the Scottish Labour Party, Ms Lamont said: "It has been an immense privilege to lead our party during this momentous time in Scottish politics.

"You will know the significant victory we campaigned for in the referendum has opened a new chapter in the debate about the future of the Scottish Labour Party.

"Some, including senior members of the party, have questioned my place in this new phase. In order that we can have the real discussion about how we take Scottish Labour forward, I believe it would be best if I took myself out of the equation and stepped down as leader.

"Anas Sarwar will take over as leader in the interim while the party elects my successor. He has been a fantastic deputy to me and I trust he is more than capable of leading the party through this transition.

"Despite the serious challenges we face, I strongly believe that the Labour Party is not only our best chance of achieving a better, fairer Scotland, it's our only chance."