"I believe leaders should walk the walk and be prepared to defend their policies," Leanne Wood, leader of Welsh nationalist party Plaid Cymru, said in another jab at May.
"How dare you call an election and run away from the debate?" Farron said later. "The Prime Minister is not here tonight. She can't be bothered so why should you?" he told voters.
Corbyn had previously said he would only join a televised debate if May agreed to do so, but he then challenged May to join him in Cambridge.
May declined, saying her priority was "getting out and about, meeting voters and hearing directly from voters". "I think debates where the politicians are squabbling among themselves doesn't do anything for the process of electioneering," May told reporters during campaigning in the city of Bath.
"The optics of Corbyn turning up to the BBC debate just as he finds momentum, and May avoiding it just as she is wobbling, will not look good [to voters]," Matthew Goodwin, a leading British political analyst, wrote on Twitter.
Goodwin was referring to recent polls showing that Labour appears to have narrowed the gap with May's Conservatives, and a surprise projection suggesting that the Conservatives could lose seats in the election next week and leave the country with a hung parliament.
The Conservatives' projected loss of about 20 seats would leave the party without an overall majority, thereby dashing May's plan to boost her numbers and strengthen her negotiating position in Britain's exit from the EU.
The forecast by polling firm YouGov for The Times - the first based on individual constituencies rather than national vote shares - puts the Conservatives 16 seats short of a majority in Parliament.
Labour, the largest opposition party, is projected to gain about 30 seats.
YouGov said it interviewed some 50,000 voters during the course of seven days, applying a modelling technique previously used to project the results of Britain's Brexit referendum and last year's US presidential election.
YouGov combined its national polling results with census and other data to produce "a fairly accurate estimate of the number of voters in each constituency intending to vote for a party on each day".
But Goodwin and other experts are not convinced by the model, pointing out that YouGov had used the same model to incorrectly project a victory for Hillary Clinton in three states where she lost in the US election.
The Conservatives had a lead of about 20 points over Labour in polls conducted in late April, but that has dropped to as little as five points in some recent polls.
An ICM poll for the Guardian on Tuesday put the Conservatives 12 points ahead.
- dpa
