At a rally as the party's spring conference kicked off in Birmingham, Mr Clegg urged voters to choose "something different".

The call came after the Lib Dems unveiled their slogan for the election, expected on May 6: "Change that works for you, building a fairer Britain."

Mr Clegg said he would not do "backroom deals" if the poll results in a hung parliament and his party holds the balance of power.

"This Government knows it's come to the end of the road. The Tories know people have started to see through them. And voters know the Liberal Democrats offer something different," he said.

"They've seen us calling it right, taking a stand, putting principles back into politics. And they believe us when we say: this is your chance for change.

"Even if you feel hopeless after everything the other parties have put you through, don't give up on change. And don't accept anything less than change that works for you."

Mr Clegg joked that the differences between the party leaders were easy to spot: "I can't throw punches like Gordon Brown and I certainly can't throw elections like David Cameron."

To laughter from delegates he added: "I bet the journalists here are still trying to work what it all means: 'Oh look, he's wearing a red tie, so that must be a signal'."

Then pointing to the lining of his jacket, he added: "Look, blue! Try and work that one out!"

Mr Clegg warned the campaign would be "tough", and Labour and the Tories would get "nasty and personal".

But he insisted this would be "the most important election in our party's history".

"On Monday morning I want you to get out there and go for broke in what will be the biggest fight of our political lives," he added.

Earlier, former senior Tory MEP Edward McMillan-Scott was unveiled as the Lib Dems' latest recruit.

Mr McMillan-Scott, once leader of the Conservative MEP faction in the European Parliament, rebelled against David Cameron's decision to break away from the centre-right European People's Party in Europe.

He was expelled by the Tories six months ago, since when he has been sitting as an independent.