Ed Miliband has been heckled by trade unionists angry over his decision not to back public sector strikes over pension reforms.
The Labour leader faced a rough ride from delegates at the TUC conference, where he delivered a keynote speech.
He told them he understood why they felt angry but said it was a "mistake" to strike while talks were going on.
Mr Miliband was also barracked as he praised the difference academies had made to education in his constituency.
In his speech, Mr Miliband stressed the strength of the link between Labour and the trade unions, which currently provide 80% of its funding.
But he also told the movement it had to change to become more relevant to the private sector and new industries - and warned that strikes should always be seen as a "last resort".
To barracking from some in the audience at Congress House, he said: "I fully understand why millions of decent public sector workers feel angry.
"But while negotiations were going on, I do believe it was a mistake for strikes to happen. I continue to believe that.
"But what we need now is meaningful negotiation to prevent further confrontation over the autumn."
In the question-and-answer session which followed, he faced a string of hostile questions about his stance on the strike by 300,000 teachers and civil servants in June.
But Mr Miliband refused to change his position - and gave a clear indication that Labour would not be backing further strikes planned for the autumn.
One delegate urged the Labour leader to "stand up on the side of hundreds of thousands of workers whose pensions are under attack" and urged him to support industrial action. Another asked him to "stick up" for public sector workers.
Mr Miliband replied: "The best thing that can be done is to avoid industrial action by a government willing to properly negotiate. That's what needs to happen.
"And what I'm going to do is stand up and say the government needs to properly negotiate with you on these issues."
Mr Miliband also rejected calls to back a return to local authority control for free schools and academies.
Struggling to make himself heard over catcalls from the audience, he said: "What you need is academies, free schools and other schools working together much better together."
He said the two academies in his Doncaster North constituency had "made a big difference" to education locally.