Setting out his agenda, which he insisted was fully funded, Mr Clegg said that he wanted to turn "anger into hope, frustration into ambition and recession into opportunity".
He accused Labour and the Conservatives of "fobbing off" the electorate with promises which could never be fulfilled.
In contrast, he said his party pledges - which include plans to break up banks, put an extra 3,000 police on the streets in England and Wales, reducing class sizes in England and give a £700 tax cut to low and middle earners - represented "hope married to credibility".
"If you have ever looked at the Liberal Democrats and thought they have got the right ideas but can they deliver, the answer is this manifesto.
"We can and we will."
The BBC's Political Correspondent Mike Sergeant said the manifesto sought to restate existing commitments and give supporters a "well-honed" message differentiating them from the other two parties.
The Lib Dems say they cannot rule out tax rises after the election in order to reduce borrowing but stress that their focus will be on cutting spending outside frontline areas.
Speaking at the same event, Treasury spokesman Vince Cable said the issue of the deficit had to be "confronted" and said Labour and the Conservatives had "banished" the subject from their manifestos.
Among their key policies, the Lib Dems are pledging to raise the state pension each year in line with earnings or inflation, whichever is highest.
Every fully trained member of the armed forces will be paid at least the same as those on starting salaries in the emergency services, they say, while political donations will be limited to £10,000 per donor.
The Lib Dems have already pledged that the first £10,000 of UK earnings would be tax-free for low and middle earners, which they will say will leave millions of people £700 a year better off and overall cost £17bn.
Raise income tax allowance to £10,000
Future tax rises not ruled out
"Mansion tax" on £2m homes
Scrap ID cards
No like-for-like replacement for Trident
£400 cap on pay rises in the public sector
£2.5bn "pupil premium" to reduce primary class sizes
By raising tax thresholds, the party wants to take about 3.4 million of the lowest-paid people out of tax altogether.
But critics have pointed out that people on incomes of up to £100,000 will also receive the same tax cut, with the Fabian Society think tank claiming that 70% of the proceeds will go to the better-off.
Mr Cable told the BBC he acknowledged better-off people would also benefit but said the wealthy would be paying more as a result of measures to raise capital gains tax, cut pensions relief for high earners and the party's "mansion tax" on properties worth £2m.
The Lib Dems said the party's spending pledges - backed up by financial tables at the back of the manifesto showing how they will be paid for by savings elsewhere - were "honest and detailed".
Among £15bn of planned spending cuts, the party would abolish ID cards, trim tax credits for high earners and not renew Trident.
Mr Clegg said he was solely focused on becoming prime minister and refused to say whether he would co-operate with either of the two largest parties in the event of a Hung Parliament.
"If there is a balanced Parliament, it is not a question of us or anybody else paving the way for someone to just have their way," he said.
"We would be in new territory where politicians would have to talk to each other. To give people a sense of what you would do if politicians needed to speak to each other, you've got to be clear about your priorities and we've been very, very clear."
Work and Pensions Secretary Yvette Cooper said there was a "multi-billion pound gap" in the Lib Dems' plans and they were "wrong" to be cutting programmes such as child trust funds.
However, Gordon Brown later said he welcomed the Lib Dems' pledge not to start to tackle the budget deficit in the current year, a policy Labour shares in contrast to the Conservatives.
"So the Liberals have agreed with us against the Tories, who want to take away this money now," he said. "It's very, very important to keep the economy moving forward this year."
The Tories focused on the Lib Dems' plans to cut income tax, saying that proposing such a "huge" tax cut was not credible given the "terrible" state of the finances.
"To be credible and get any proper consideration, they have to come forward with serious proposals about how they are going to fill the gap in the public finances," Shadow Chief Secretary to the Treasury Philip Hammond told Sky News.
"The numbers don't add up."
And Conservative leader David Cameron urged those wanting "a fairer, greener and more liberal Britain" to support the Conservatives as the party able to get "things done" in government.
The Scottish National Party (SNP) and Plaid Cymru said the Lib Dems were offering "more of the same" as Labour and the Conservatives.
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