The Liberal Democrats are the only credible opposition to the Tories because Labour's leftward slant under Jeremy Corybyn will alienate voters, Tim Farron has told his first party conference as leader.
Mr Farron hinted that the Lib Dems could enjoy a similar surge in support to that which led to the Scottish National Party's electoral success.
He pinned his hopes for a Lib Dem revival on offering a new home to disaffected Labour members and voters distressed at the election of Mr Corbyn as leader.
The Lib Dems were left reeling in May as their 56 MPs were reduced to a rump of eight, triggering Nick Clegg's resignation as leader following five years in government.
In his first conference appearance as leader, Mr Farron attempted to rally the party's activists, insisting that the surge in membership since the election - with more than 20,000 people signing up - was the sign of a fightback.
Mr Farron, who condemned the "fantasy economics" offered by Mr Corbyn and shadow chancellor John McDonnell, said: "We once again see the prospect of a decade or more of Tory rule, and it fills us with dread."
He told activists: "If Labour aren't interested in standing up to the Tories and providing a credible opposition, that's their funeral.
"The Liberal Democrats will fill that space. Radical and liberal and responsible too."
At the conference's opening rally in Bournemouth, Mr Farron said: "When the tectonic plates of politics move, they sometimes move immensely quickly - that is what is happening now.
"These are momentous and historic times, history calls us, we will answer that call.
"Britain needs a party that is progressive, moderate and liberal. We are that party. This is our moment."
Mr Farron, who has been in contact with disaffected Labour figures since Mr Corbyn's election, played down suggestions of defections to his party, saying "we are not talking about that at all".
But he added: "What I am saying is a number of people have spoken to me, shall we say, because I think there is a great fear across the progressive grouping in British politics that something has happened recently which makes the Tories' stay in power look a lot more secure and a lot more lengthy.
"And that is bad for Britain and bad for progressive politics, and I'm not having that. So I am determined that the Liberal Democrats will take that opportunity to be that liberal, progressive, economically responsible force.
"That is the only way the Tories will be removed."
Despite his party's reduced presence in the Commons, Mr Farron pointed to the rapid change in fortunes of the SNP.
He said:"The SNP had six MPs six months ago and it's now a one-party state up there. I'm not seeking that the Liberal Democrats form a one-party state, I am saying don't write us off."