Liberal Democrat leader Nick Clegg said the Conservative manifesto had been launched in a power station that "no longer generates power. It's style over substance".
He said his own party's blueprint for power would not promise "something for nothing", unlike both Labour and the Conservatives.
Speaking on the campaign trail in Bradford, Clegg said: "You can't trust the Conservatives. They have just launched a manifesto in a power station that doesn't generate power.
"It's a manifesto of style over substance."
Gordon Brown said the 130-page document offered nothing to support the economic recovery and would leave people to fend for themselves.
Speaking on the campaign trail in Derby, he said: "There is a complete hole at the centre of the Conservative manifesto.
"There is nothing in it to help the recovery. Indeed their measures would put the recovery at risk."
He added that the manifesto showed that the Conservative Party "hasn't changed, despite all the rhetoric".
One of the authors of Labour's manifesto, international development secretary Douglas Alexander, tweeted: "The Tories set out an agenda this morning not to empower but to abandon people and leave them to face change alone."
Clegg said the Tories could not be trusted when they want to give "tax breaks to double millionaires not tax breaks to everybody else".
"You can't trust the Conservatives when they don't want to clamp down on the bonuses of greedy bankers," he said.
"You can't trust the Conservatives when they don't really want to clean out the corrupt state of politics in Westminster."
He added: "David Cameron seems to think that it's just his turn to govern, that he should just inherit power rather than earn it."
Commenting on today's manifesto launch by the Tories, Nick Clegg's chief of staff, Danny Alexander said Cameron "only offers fake change".
He said the truth is you "cannot trust the Conservatives" and that the party leader believes it is "his turn to take over in the same way the two old parties have taken turns for years".
"Today's manifesto offers the same old empty promises based on the lazy assumption that people will just give them another go," he said.
"When the Tories say we're all in this together, what they really mean is you're on your own. Their agenda is to take away help from those who need it and offer it to those already at the top.
"The Tories won’t make life better for ordinary people. Their manifesto offers only fake change not the real change this country desperately needs."
The party will launch its manifesto on Wednesday.
