Senior Labour politicians have led tributes to Gordon Brown and voiced pride in the party's achievements in 13 years in power.
Education Secretary Ed Balls - a close ally of Mr Brown since Labour's last period in opposition - told a news programme: "Obviously, it is a sad day for the Labour Party, but at the same time Gordon Brown can go out with pride in his great achievements in government over 13 years.
"It's the end of an era. It's the end of the Gordon Brown era."
Mr Balls said it was a "disappointment" for Labour that the Liberal Democrats had shown themselves willing to go along with Conservative plans to reduce state spending immediately, rather than maintain it until the recovery is firmly established.
Former Home Secretary David Blunkett said: "This is a sad moment and one in which to spare a thought for Sarah and the two young boys. It is now the moment for the Labour Party to reflect on what has happened - but, far more importantly, what the future holds.
"There can be few tasks in life as thankless as being Prime Minister. This is true in spades when dealing with a global recession in which everywhere else in the world is affected - but your own electorate have no-one else to blame but you.
"That is why everyone, including those who have been vitriolic about Gordon Brown, should have the decency to say 'Thank you' for his tremendous contribution to getting nations to work together over the last two years to combat what would have been a global depression."
Welsh Secretary Peter Hain said: "Whilst the Liberal Democrats are fully entitled to link up with the Tories, I am sorry that they rejected the opportunity for a progressive coalition government to fundamentally change British politics.
"We will be taking our role as the Opposition very seriously, and holding the Conservatives and Lib Dems to account if they fail to deliver what the country needs and working with them where there is common ground.
"I am proud of Labour's many achievements in government. We have transformed public services and public infrastructure and there are two million more jobs in Britain. We will be using every opportunity to try to prevent Conservative plans for savage and early cuts in public sector jobs and services."