Jeremy Corbyn has clashed with Home Secretary Amber Rudd over spending cuts in a seven-way TV election debate.
Mrs Rudd is representing the Conservatives after Theresa May declined to take part.
She repeatedly accused Mr Corbyn of having a “magic money tree” after he highlighted a Tory U-turn on disability benefits and accused the party of plotting five more years of austerity.
Mr Corbyn did not criticise Mrs May’s absence in his opening statement.
But Lib Dem leader Tim Farron did take a swipe at the Tory leader, saying: “Where do you think Theresa May is tonight?
“Take a look out your window. She might be out there sizing up your house to pay for your social care.”
Plaid Cymru leader Leanne Wood said Mrs May was not there because “her campaign of soundbites is falling apart.”
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In her opening statement, Amber Rudd repeatedly hailed Theresa May’s leadership qualities and her “record of delivery”.
UKIP’s Paul Nuttall, SNP deputy leader Angus Robertson and Green co-leader Caroline Lucas are also on the panel.
Mrs May said she was taking questions from voters around the country instead of “squabbling” with politicians in a TV debate, after Mr Corbyn made a surprise announcement earlier that he planned to take part.
Mrs May has already ruled out taking part in head-to-head debates, and Labour had said that Mr Corbyn would not attend unless she was there.
But on the afternoon of the event, Mr Corbyn confirmed he would take part, and criticised the Tories for what he called “a stage-managed arms-length campaign”.
“Refusing to join me in Cambridge tonight would be another sign of Theresa May’s weakness, not strength,” he added.
Taking questions during a campaign visit in Bath, Mrs May said Mr Corbyn “seems to be paying far more attention to how many appearances on telly he’s doing, and he ought to be paying a little more attention to thinking about Brexit negotiations”.
Asked whether she was frightened of taking on Mr Corbyn, she said she had been doing this every week during Prime Minister’s Questions, adding that it was “so important” to be taking questions from voters.
“That’s why I’ve been doing that up and round the country,” she added.
But Lib Dem leader Tim Farron accused her of “keeping voters in the dark”.
“Theresa May called this election but now won’t even turn up to debate the issues,” he said.
Mishal Husain is moderating the debate, which takes place in Cambridge and being shown on BBC One from 19:30-21:00 BST and livestreamed on Twitter.
The show is the latest in a series of special broadcasts ahead of the 8 June general election.
This includes two Question Time shows – on 2 June featuring Mrs May and Mr Corbyn appearing separately and on 4 June with Mr Farron and SNP leader Nicola Sturgeon.
Andrew Neil has been carrying out a series of interviews with party leaders.