Nick Clegg has won the race to become the next Liberal Democrat leader.
The 40-year-old beat Chris Huhne to become the party's third leader in two years - in a contest which turned out to be even closer than expected.
Mr Clegg, an ex-journalist and former Euro MP, won 20,988 votes to the 20,477 votes cast for Mr Huhne by members.
In his acceptance speech Mr Clegg said he wanted his leadership to be about "ambition and change", saying "we want to change politics and change Britain".
The Sheffield Hallam MP said he wanted to mark the "beginning of Britain's liberal future".
Mr Clegg, speaking after the announcement at a central London hotel, acknowledged it had been a "close fought contest" and praised his rival, who he said he was looking forward to working with.
He said he wanted to attract those voters who shared liberal values, but did not currently vote for the party.
He thanked Vincent Cable for a "spellbinding" performance as acting leader - and praised his predecessor Sir Menzies Campbell, whose resignation in October sparked the leadership race.
Mr Clegg, who has two children with wife Miriam, a Spanish lawyer, said that without Sir Menzies, the party would not have the "bright future" it now faced.
He said Labour and the Conservatives were "mutating" into each other, and urged disaffected voters to join the Lib Dems, saying he wanted to "provide a liberal alternative to the discredited politics of big government".
He pledged to spend at least a day a week campaigning outside Westminster, and to hold regular public town hall meetings to give people who were not Lib Dem members, but supported the party, a chance to have their say.
He accused the Conservatives of having "no answers to the big issues" and said Labour was "increasingly exhausted and discredited" - saying it left an opportunity for the Lib Dems.
"I want the Liberal Democrats to be the future of politics, because Liberal Democrats have the courage to imagine a better society to break the stifling grip of the two party system for good.
"To bring in a new politics, of politicians who listen to people, not themselves. No more business as usual. No more government knows best.
"I want today to mark the beginning of a real change in Britain. The beginning of Britain's Liberal future."
Mr Huhne thanked everyone involved in his campaign and said he looked forward to working with Mr Clegg - whom he praised for his warmth, intelligence and energy.
He told the BBC: "I have been around the course long enough that I know what the rules of the game are - you win some you lose some, it was a close-run thing."
He said there was inevitably an "in-built abrasiveness" to leadership contests, but said the contest had got the party's message across and he was sure he and Mr Clegg would go on working well together.
Mr Clegg is the party's third leader in two years - Charles Kennedy quit in January 2006, forced out by a frontbench rebellion after admitting a drink problem. Sir Menzies resigned in October, blaming an age-obsessed media.
Following the announcement, Mr Kennedy told the BBC: "I think that, you know, barring acts of God, this will be our last leadership contest for a very long time. I certainly hope so anyway. And you know, I think we've now got fresh faces, a fresh start, and that's good news."
Sir Menzies said Mr Clegg had shown in his speech he had the drive and ability to take the party forward, adding: "It's a day for looking forward, not for looking back."