Tim Farron has pledged to double the number of Liberal Democrat MPs at the general election following his party’s performance in the council polls.
Mr Farron said the party had increased its share of the vote by 7% – but it lost more than 40 seats.
He said the Conservatives were headed for an election landslide and warned only the Lib Dems stood in the way of a “one-party state”.
In 2015, the Lib Dems ended up with eight MPs, down from 57 in 2010.
In the UK’s local elections, the Conservatives have made big gains at the expense of Labour and UKIP, with the party winning control of 11 new councils in England and Wales so far.
Speaking during a campaign visit to St Albans, Hertfordshire, Mr Farron said Labour had “utterly imploded”, while the Conservatives were on course for a majority in the 8 June general election.
“Imagine this, on the ninth of June with a majority larger than Margaret Thatcher’s, imagine what that means for your family, what it means for you, what it means for your job security, what it means for your hospital, for your school,” he said.
Mr Farron said he had grown up under a government with a colossal majority that had taken the country for granted.
“I want to lead a country that says that is not acceptable,” he said.
Party president Baroness Brinton said the Lib Dems were now “breathing down Labour’s necks” with a projected national vote of 18% against 27% for Jeremy Corbyn’s party.
“With five weeks of the general election campaign to go, we are just 9% behind Labour and breathing down their necks,” Lady Brinton said.
“We have halved the gap on Labour in just one night and scored our best national election result in seven years.
“Labour is collapsing and we will stand up for people to provide the strong opposition this country needs.”
Labour lost ground in Lincolnshire, Cumbria and Warwickshire and Phil Johnson, the party’s general election candidate in Nuneaton, lost his seat on Warwickshire council to the Conservatives.
Ex-MP Stephen Kinnock, who is standing again in Aberavon, said the party’s performance was “pretty disastrous”.
But shadow chancellor John McDonnell said this was not the “wipe-out that many commentators were forecasting” and Labour was building a “solid base” for the general election.
Polling expert John Curtice told the BBC that the Lib Dems “have made progress” but were not doing as well as they did locally before the coalition.
The BBC’s projected national vote share for the Tories is 38% to Labour’s 27%, the Lib Dems’ 18% and UKIP’s 5%.
Conservative leader Theresa May sought to combat any complacency in her party’s ranks by saying she was “taking nothing for granted” in the general election.