Jeremy Corbyn insisted he would take a "neutral stance" in an EU referendum under Labour while Boris Johnson was challenged over "racist rhetoric" and trustworthiness during a brutal televised grilling.
The Prime Minister refused to apologise over his use of language and defended Tory austerity during a special episode of BBC's Question Time on Friday.
The Labour leader also came under fierce scrutiny from voters when he was questioned over fears for businesses, anti-Semitism, misogyny, freedom of speech and Scottish independence.
As they try to tip the balance in the campaign for the December 12 General Election, the leaders of the four main parties were quizzed for half-an-hour apiece.
But the highest levels of groaning and jeering were reserved for the frontrunners in the Sheffield studio.
Mr Corbyn made his clearest comment to date on how he would act in another referendum, which Labour plans to hold between a new deal and the option to Remain within six months of taking power.
Having been asked how anyone could vote Labour without knowing what outcome he would campaign for, Mr Corbyn said that he would start by negotiating a "credible" Leave deal before he was interrupted by laughing.
"And I will adopt, as prime minister, if I am at the time, a neutral stance so that I can credibly carry out the results of that to bring communities and country together rather than continuing an endless debate about the EU and Brexit," he continued.
Host Fiona Bruce pressed Mr Corbyn on whether he would not pick a side during another referendum, as she brought his time to a close.
"Yes," he replied. "First heard here on Question Time."
The Tory PM was asked to apologise and admit he had personally contributed to "racist rhetoric" during his journalistic work.
Ms Bruce challenged him for comparing veiled Muslim women to "letterboxes", referring to "watermelon smiles" and "flag-waving piccaninnies" and to "tank-topped bum boys".
Mr Johnson said: "I have written many millions of words in my life as a journalist and I have... genuinely never intended to cause hurt or pain to anybody and that is my intention.
"What I will say because I think you are referring to a particular article of a year or so ago..."
The audience scoffed after Mr Johnson said: "If you go through all my articles with a fine-tooth comb and take out individual phrases, there is no doubt that you can find things that can be made to seem offensive and of course I understand that."