She was accused of lacking “guts” and of “running away” from the 90-minute BBC debate. The prime minister ruled out appearing on televised debates soon after calling the election.
She said on Wednesday that she preferred “taking questions and meeting people” on the campaign trail rather than “squabbling” with other politicians.
Opposition Labour leader Jeremy Corbyn did not directly attack Mrs May’s absence as he clashed with Home Secretary Amber Rudd, who represented Mrs May’s Conservative Party.
Ms Rudd said “part of being a good leader is having a good, strong team”.
At one point, Mr Corbyn asked “where is Theresa May, what happened to her” as he defended his own leadership abilities.
Ms Rudd said the “squabbling” on display during the debate showed “the coalition of chaos in action”.
She said Mrs May had the strength to “take us through Brexit” and a vote for anyone else was a vote for Mr Corbyn “and that coalition”.
Lib Dem leader Tim Farron launched a string of attacks on Mrs May. He said: “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.”
Green party co-leader Caroline Lucas said the “first rule of leadership is to show up”.
She added: “You don’t say it’s the most important election of our lifetime and not be bothered to show up.”
Plaid Cymru leader Leanne Wood said Mrs May was not there because “her campaign of soundbites is falling apart”.
SNP deputy leader Angus Robertson accused Mrs May of not having the “guts” to attend the debate, as he attacked Ms Rudd over cuts to the winter fuel allowance for pensioners in England.
Labour said for weeks that Mr Corbyn would not attend the debate unless Mrs May was there – but he announced at lunchtime that he would take part.
Mr Corbyn engaged in a series of clashes with Ms Rudd over the squeeze on living standards and cuts to welfare as the debate heated up.
“Have you been to a food bank, have you seen people sleeping around our stations,” he asked Ms Rudd.
“Have you seen the levels of poverty that exist because of your government’s conscious decisions on benefits?”
Mr Corbyn also highlighted his plans to end the public sector pay cap and introduce a £10 an hour living wage by 2020.
He said Labour would “ensure our manufacturing industry and jobs are protected”, and he was “absolutely sure” his spending plans added up.
Ms Rudd said the Conservatives had “made a clear decision to protect the poorest in our society,” adding that “winter fuel payments won’t be available for billionaires” under her party’s policy.
She dismissed her rivals’ claims as “fanciful”, saying they offered nothing but “bluff, bravado and tempting, shiny election promises”.
“The only question to consider is who should be in No 10 to steer Britain to a brighter future?” Ms Rudd asked.
“Jeremy Corbyn with his money tree, wish list manifesto and no plan for Brexit – or Theresa May with her record of delivery.”
The Greens’ Ms Lucas said she wanted to “make the case” for freedom of movement across the EU and the ability of people to be able to “live and love” in other countries.
“The Britain I love is a confident outward-looking country,” she said, which well knows the “benefits” of migration.
UKIP leader Paul Nuttall denied claims he was demonising immigrants, but said: “We have to get the population under control.”
The panel members also debated security and terrorism, the NHS and US President Donald Trump’s decision to pull the US out of climate change agreements.
Mr Corbyn criticised the Tories for what he called “a stage-managed arms-length campaign”.
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”.
She said she had taken on Mr Corbyn 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.