Labour's new leader Jeremy Corbyn faced an exodus of senior figures from the party's front bench team within minutes of being elected as successor to Ed Miliband.
The veteran left-winger won a landslide victory with almost 60% of more than 400,000 votes cast by Labour members and supporters, trouncing rivals Andy Burnham, Yvette Cooper and Liz Kendall in the first round of counting.
In an acceptance speech to cheering supporters at Westminster, he called repeatedly for "unity" and announced his ambition to lead a Labour "fightback", declaring: "We don't have to be unequal, it doesn't have to be unfair, poverty isn't inevitable, things can and they will change."
Mr Miliband offered the new leader his support and called on others in the party to do the same, but made clear he will remain on the backbenches.
Shadow home secretary Ms Cooper said she did not expect to take a shadow cabinet post, while shadow work and pensions secretary Rachel Reeves announced she would go to the backbenches when she returns from maternity leave.
Shadow communities secretary Emma Reynolds and shadow health minister Jamie Reed also said they would not serve under Mr Corbyn.
Ms Kendall, shadow education secretary Tristram Hunt and shadow chancellor Chris Leslie had already indicated during the three-month leadership campaign that they would leave the front benches if the Islington North MP won.
However, shadow business secretary Chuka Umunna - a senior centrist who withdrew from the contest after a matter of days - issued a plea for the party to "come together" behind its new leader.
In a post on Facebook, Mr Umunna said: "Now the contest is over, we must respect the result, come together and focus on providing the most credible and effective opposition to the Tories."
Mr Corbyn's victory was hailed by supporters who chanted "Jez we did" as he took to the stage of the Queen Elizabeth II conference centre. Earlier, some of them sang the Red Flag to greet him as he arrived to hear the result.
There were loud celebrations in the hall as the party's National Executive Committee chair Jim Kennedy announced that Mr Corbyn had taken 59.5% of the vote - 251,417 of the 422,664 votes cast - against 19% for Mr Burnham, 17% for Ms Cooper and 4.5% for Ms Kendall.
The new leader has spent his entire 32-year parliamentary career on the backbenches and entered the leadership contest as a 200-1 outsider before generating a wave of enthusiasm which swept him to victory.
He now faces the immediate challenge of facing David Cameron across the despatch box at Prime Minister's Questions on Wednesday, as well as constructing a shadow cabinet which will deliver his anti-austerity, anti-war policies without splitting the party.
The scale of his triumph - which saw him take clear majorities among not only the supporters affiliated through the unions and those who paid £3 to become registered supporters, but also full members - will make it extremely difficult for any disgruntled moderates to mount a bid to unseat him before the 2020 general election.
Accepting his new post, Mr Corbyn said the campaign "showed our party and our movement, passionate, democratic, diverse, united and absolutely determined in our quest for a decent and better society that is possible for all".
He went on: "During these amazing three months, our party has changed. We have grown enormously, because of the hopes of so many ordinary people for a different Britain, a better Britain, a more equal Britain, a more decent Britain.
"They are fed up with the inequality, the injustice, the unnecessary poverty. All those issues have brought people in in a spirit of hope and optimism...
"The fightback now of our party gathers speed and gathers pace."
He announced his first act as leader will be to attend a "Refugees Welcome Here" rally, and his first day at the helm of his party in Parliament will be spent opposing Government plans to "shackle" trade unions by imposing higher thresholds for strike ballots.
Union leaders welcomed his victory. Unite general secretary Len McCluskey said "voters can now look at Labour and see, unquestionably, that it stands for fairness, justice, peace and strong communities".
Unison general secretary Dave Prentis said: "?Jeremy has ignited a spark of hope, a spark that had been dampened for decades. This is a chance to claim back the heart and the soul of the party and make it our Labour Party once more?."
For the Conservatives, Defence Secretary Michael Fallon said: "Labour are now a serious risk to our nation's security, our economy's security and your family's security.
"Whether it's weakening our defences, raising taxes on jobs and earnings, racking up more debt and welfare or driving up the cost of living by printing money - Jeremy Corbyn's Labour Party will hurt working people.
"This is a very serious moment for our country - the Conservatives will continue to deliver stability, security and opportunity for working people."
In a statement, Mr Miliband said: "I offer Jeremy Corbyn my support in what is a very difficult and demanding job, and I hope that people across the party will do the same.
"At the same time, I hope and expect that Jeremy will do everything he can to reach out and use the talents of people right across the party in the task of taking on the Tories and facing up to the very big challenges that we face.
"For my part, I will be supporting his work by advancing the causes I care most about - including tackling inequality and climate change - from the backbenches."
Former deputy prime minister Lord Prescott urged high-profile figures to "get behind" Mr Corbyn, warning that the "real enemy is the Tories".
He told the BBC: "The party gave an overwhelming endorsement to this man. He got more votes than Tony Blair.
"The party has spoken in a very, very strong voice."
West Bromwich East MP Tom Watson was elected deputy leader, and is expected to play an important role in helping to build Mr Corbyn's frontbench team and maintain party unity and discipline.