Tim Farron declared his door was "wide open" to potential Labour defectors amid warnings Jeremy Corbyn's party would be "slaughtered" by the Conservatives.
The Liberal Democrat leader refused to rule out a power-sharing deal with Mr Corbyn's Labour but made an open pitch to the party's members and elected politicians to jump ship.
Mr Farron also appealed to moderate Tories with concerns about David Cameron's "un-British" attitude to the migrant crisis to join his party.
He hopes that he will be able to take advantage of a "vast space" in the centre ground caused by Labour's election of left-winger Mr Corbyn and Mr Cameron's policies since taking office without the restraints imposed on him by the Lib Dems.
Despite only having eight MPs after a disastrous general election, Mr Farron insisted his party was the only "credible" opposition to the Tories.
Mr Farron used an appearance answering questions on the Lib Dem conference stage in Bournemouth to attack Mr Cameron's "increasingly unpleasant, illiberal, frankly un-British rhetoric about refugees" and his "utter short-termism over the EU referendum".
Turning to Mr Corbyn's Labour he said the "desire to move even further away from economic credibility undermines their ability - anybody's ability - to be able to provide social justice, equality and fairness".
He said: "I am not a home wrecker for Labour MPs, but I am a home builder ... My door is wide open to liberals and social democrats who are not yet members of the Liberal Democrats. This is a very bold and open offer to those people.
"I'm not just talking about Members of Parliament and councillors and lords and members of the Scottish Parliament and the National Assembly of Wales, the Greater London Assembly and the European Parliament and all those parliamentarians and elected people.
"I'm talking about the massive majority of people in this country who are voters but aren't elected to any particular office, many of whom consider themselves to be supporters of the Tories or perhaps Labour.
"To you, if you are in your heart a liberal or a social democrat, you have a home in the Liberal Democrats."
Following speculation about possible defections, Mr Farron would not reveal how many Labour MPs suffering from "angst" since Mr Corbyn's election had approached him.
He told Sky News's Murnaghan programme: "When all is said and done, it's not fair that I put those people under pressure.
"Being human and ignoring the politics for a moment, the angst of many people who have worked in the Labour Party for years and feel that it has dramatically changed into something they don't believe in in the last fortnight is real and it is a human issue and I think we should have empathy and sympathy.
"It's not for me to go making their lives any harder."
In an interview on BBC Radio 5 Live Pienaar's Politics programme the Liberal Democrat leader refused to confirm whether he would be happy to work with Mr Corbyn's Labour - insisting this was a matter of "arithmetic" following an election because preferences are "redundant".
Former Cabinet minister Sir Vince Cable said he believed Mr Corbyn's leadership had left Labour "completely unelectable".
The ex-business secretary said: "The Tories are absolutely ruthless, as we know to our cost. They will slaughter them."