Former Scottish National Party leader Alex Salmond has joined the race to replace his successor, John Swinney.
Alex Salmond entered the race to take over from John Swinney
The Banff and Buchan MP, who had earlier ruled out a leadership bid, said he had been persuaded to stand by the party's grassroots members.
He told a news conference in Aberdeen that he was making the move "with a degree of surprise and humility, but with a renewed determination".
Nicola Sturgeon dropped her leadership bid and will seek to become his deputy.
Mr Salmond said that if their campaign was successful, Ms Sturgeon would become the party's leader in the Scottish Parliament.
MSP Kenny MacAskill has decided not to stand for the deputy leadership post and has thrown his weight behind Mr Salmond and Ms Sturgeon.
Mr Salmond telephoned campaign managers for the other candidates on Wednesday to tell them of his intention to run for leader again.
The party's leader at Westminster, he had repeatedly denied that he would seek the job which he had left four years ago.
In June, he said: "If nominated I'll decline. If drafted I'll defer. And if elected I'll resign."
Thursday's news conference was held in the same venue where he declared in 2000 that he was standing down as leader.
"I did not expect to be ever doing the job again, however time and circumstances change," he said.
"I did not anticipate that after waiting 300 years for a Scottish Parliament that it would allow itself to sink so swiftly into something approaching disrepute.
"I did not expect that such a failing Labour Party with such mediocre leadership would have been able to cling to power in the face of such a changing political landscape.
"And I did not anticipate that after the rebirth of Scottish confidence and democracy that we should have made such pitiful progress as a nation."
Mr Salmond said that Labour's "free run" north of the border was over, declaring: "I shall make them dance to a Scottish tune."
He said he planned to lead the SNP to progress at next year's general election and to government at the next Holyrood elections, and to offer the Scottish people the chance to move towards independence.
"Let me be clear, from today I am not just launching a campaign to be SNP leader," he said.
"Today I am launching my candidacy to be the first minister of Scotland."
Mr Salmond said that over the last few weeks a "huge number" of SNP members had contacted him asking him to stand.
"It is their intervention and theirs alone which finally convinced me to reconsider my decision and seek office," he added.
Ms Sturgeon said she believed she was capable of filling the "vital" role of leading the SNP group at Holyrood.
Nominations close 16 July
Ballots mailed to all members by 13 August
Ballots to be returned by 31 August
Result announced 3 September
"I am confident that, together, Alex and I will be the winning team that puts the SNP back on track and Scotland on the road to independence," she said.
Mr MacAskill said: "I believe that the Salmond/Sturgeon team is the best to unite the party and take the SNP forward."
The announcement came on the eve of the close of nominations.
'Won't be a doormat'
The other contenders for the leadership, which will be decided in September, are former MSP Mike Russell and MSP Roseanna Cunningham, the current deputy leader.
Mr Russell said he was surprised by Mr Salmond's decision, but added he was not afraid to engage in policy debate with the former SNP leader.
Ms Cunningham said Mr Salmond's about-turn would not mean she would pull out of the leadership race.
She said: "Nothing that has happened this week has changed my mind.
"I'm certainly not going to be a doormat for anyone else and in my view it will be a matter entirely for the party to decide whether or not it wants to go back with Alex Salmond or into the future with me."
Mr Salmond stepped down at the party's annual conference in 2000 after 10 years as national convener.
The 49-year-old was an economist with the Royal Bank of Scotland before winning his Westminster seat from the Tories in 1987.