The Liberal Democrats would "add a heart to a Conservative government and a brain to a Labour one", Nick Clegg said on Wednesday at the launch event for the party's manifesto.
Clegg used the event in Battersea to tell Britons why they should vote for the Lib Dems, and said the party would offer stability to a Conservative or Labour government.
"We won't allow the Conservatives to cut too much and jeopardise our schools and hospitals, and we won't allow Labour to borrow too much and risk our economy," he said.
He said a government that included UKIP's Nigel Farage or former SNP first minister Alex Salmond would be disastrous for Britain.
"Imagine for a moment what will become of our wonderful country if Nigel Farage and his friends on the right wing of the Conservative party are calling the shots," he said. "Our public service cut to the bone, our communities divided, our shared British values of decency, of tolerance, of generosity, cast aside.
"And now imagine a Britain run by Ed Miliband and Alex Salmond... Our economy crippled by reckless borrowing, our children destined to pay for it for years to come. the future of our United Kingdom in the balance once again."
Clegg added that "every vote for the Liberal Democrats" would count because of the likelihood of a hung parliament as polls increasingly suggest that no party will win an overall majority, and that the way to keep Salmond out was for people in the Gordon constituency to vote for the Lib Dems' Christine Jardine instead.
Salmond was quick to respond to the criticism.
"I'm delighted to get so many mentions at the Lib Dem manifesto launch this morning," he said.
"Of course people should know why Nick Clegg namechecked his candidate for Gordon. ... He was trying to make amends for forgetting her name in a recent interview with the Press and Journal in Aberdeenshire.
"Who has ever heard of a party 'leader' trying to undo such a disaster at his own manifesto launch? Little wonder he's known as Calamity Clegg by his own party."
The venue for the launch also gained considerable attention. One journalist had previously gone raving there.
Not saying I'm depressed at life's journey, but I once attended a rave in the space in which I'm now waiting for the LibDem manifesto launch
Is it a night club ? A power failure ? No. It's the @LibDems manifesto launch #hipsville
Others were just a little confused.
Unsure as to whether I'm at a manifesto launch or someone's birthday party
Journalists at the event got very animated about Lib Dem members – who obviously support the party – being given the chance to ask more questions than journalists.
Worst launch press conference yet by Lib Dems one question from the FT and the rest dolly drops by party members. Pathetic.
Bizarre that Clegg - veteran of @LBC phone-ins and monthly press conferences - takes ONE question from a journalist #LibDemManifesto
But that's not to say everyone invited was paying loads of attention. One guy decided to play chess on his phone.
There's one LibDem here who's so interested in events that he's playing chess on his mobile. Not quite Angry Birds