At least 60 campaigners and their legal team have walked out of the inquiry into undercover policing demanding that its chairman, Sir John Mitting, stand down or appoint a full panel for the rest of the hearings.
There has been growing discontent among core participants at the inquiry, which started in 2015 and was due to finish this year, but has been beset by delays.
Phillippa Kaufmann QC, representing the group, told the Royal Courts of Justice on Wednesday: "It is now abundantly clear that we simply cannot participate in this hearing in a meaningful way."
The campaigners have raised concerns about former undercover officers being granted anonymity during the proceedings, and also the slow speed of progress.
Ms Kaufmann claimed that the chairman's reasoning for not releasing the names of certain police officers was "scant and largely uninformative".
She said her clients "therefore ask that you recuse yourself of this enquiry or that you bring about a new panel.
"I'm instructed with the entire legal team to withdraw while these issues are considered by you. So that is all I have to say this morning."
Her words were greeted by cheers and applause inside the courtroom, followed by the vast majority of the public gallery streaming out.
The inquiry, which was launched in March 2015, has been delayed partly because the original chairman Sir Christopher Pitchford died, and is due to take at least another year before evidence is heard.
A woman known as Cathy, from campaign group Police Spies Out of Lives, said: "We've all been disappointed at the slow rate of progress and the complete lack of disclosure."
She claimed that Sir John, who took over the role as chairman last summer, "seems to give more weight to the privacy of the officers" than those who were affected by the undercover activity.
"There are two key questions. What were the cover names, and which groups were spied on? Without those two bits of information it's nigh-on impossible for people to come forward to say this is what happened to me and my group," she said.
The campaigners would like to see an inquiry led by a panel rather than a single judge, because they believe that would better investigate claims of institutional racism and sexism.
At a hearing last month, the participants were told they had hit a "brick wall" in terms of access to certain information. They stood outside the Royal Courts of Justice on Wednesday with a banner emblazoned with a brick wall, chanting "Mitting out".
Blacklisted construction workers and the Blacklist Support Group have been granted "core participant status" in the inquiry because of undercover police infiltration of trade unions.
Spokesman Dave Smith said: "Blacklisted workers who have been kept under surveillance by political policing units were always sceptical about whether the British state investigating itself would truly provide justice.
"Time and again (chairman Sir John Mitting) gives the police the benefit of the doubt, to the detriment of those whose lives have been torn apart by this human rights scandal.
"Tinkering around the edges isn't going to change things. We have no confidence in Mitting. He must go and needs to be replaced with a panel of experts."