David Cameron's response to the migrant crisis has been "pitiful and embarrassing", Tim Farron claimed as he put the issue at the centre of his first keynote conference speech as Liberal Democrat leader.
He said the UK should opt in to the European Union plan to resettle refugees who had made perilous trips to the continent, warning that "if we don't act now, many more will die".
Mr Farron seized on the issue as he made his pitch to the public - and Lib Dem activists still reeling from the disastrous general election - that his party should return to government in 2020.
In his speech in Bournemouth he said his party should be "serious about power" and claimed the election of Jeremy Corbyn as Labour leader had left the Lib Dems as the only credible opposition to the Tories.
"Against all the odds, we have just been given the chance to take centre stage," he said.
The Lib Dems were reduced to a rump of just eight MPs as voters turned their back on the party after five years in power with the Tories.
But Mr Farron told activists they should be prepared to return to office rather than retreat to the comfort zone of opposition.
"If others wish to abandon serious politics, serious economics, that's their lookout," he said.
"But you can be certain that the Liberal Democrats will occupy every inch of that progressive liberal space because you cannot change people's lives from the glory of self-indulgent opposition."
Looking ahead to elections in Wales, Scotland, local councils and London next year he said "our mission is to put Liberal Democrats back in power at every level throughout Britain".
Mr Farron, who visited Calais shortly after becoming leader to see the effects of the migrant crisis for himself, reserved his strongest language for a section on the plight of the refugees seeking a new life in Europe.
He said: "I met a 14 year-old boy who had broken both of his legs trying to board a lorry. He was in a wheelchair pushed by a boy who was 11. Both had lost their parents, both were alone.
"And I realised that the UK Government was ignoring their humanity, it was just stuck in media management mode, following not leading.
"And the Government is still following the story. It's just a rather different one.
"It's the body of a three year old boy face-down in the surf. And what we've had from David Cameron is a careful calibration of what it will take to manage that story, the minimum effort for the maximum headlines.
"And a policy which will not directly help a single one of the hundreds of thousands currently on the move across Europe.
"It's pitiful and embarrassing and makes me so angry."
Mr Farron called for the UK to accept a quota of migrants as part of the EU plan which Mr Cameron has opted out of, although Lib Dem aides would not give a figure for the number of people the country should be prepared to accept.
Referring to the arrival of refugees before the Second World War and the country opening its doors to Ugandan Asians, Mr Farron said: "I realise how much richer - culturally, socially, economically - our society is today, because of our generosity then.
"What a lesson in seeing the best in people and not the worst. What a lesson in liberalism. As the party of outsiders, we will stand up for the outsiders."
He added: "Winter is coming and the risks and hardships faced by those seeking sanctuary will only increase.
"If you are shocked by the pictures on our TV screens today, just think how much worse they will look when the snows come to the Balkans.
"If we don't act now, many more will die."
Senior Lib Dems hope taking a lead on the migration crisis will appeal to liberal-minded people across the country, in a similar way to former leader Charles Kennedy's stance on opposing the Iraq War.