Labour MP Ruth Smeeth has been escorted by colleagues to a hearing where she will give evidence against a party member accused of anti-Semitism.
Luciana Berger said she and others were there in "solidarity" and they wanted to see "strong action" over the issue.
Marc Wadsworth was suspended from the party after a 2016 incident. He said he was "confident" of being cleared.
Labour has promised to settle the "vast majority" of the outstanding cases of alleged anti-Semitism by August.
After meeting members of the Jewish Leadership Council and the Board of Deputies of British Jews on Tuesday, Jeremy Corbyn said dealing with complaints swiftly and fairly was a top priority for Labour officials.
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But the Jewish leaders said the talks had been a "disappointing, missed opportunity" to resolve longstanding problems within the party and did not agree a minimum plan of action.
As Mrs Smeeth entered the building in central London where the hearing is being held, a group of protestors chanted "reinstate Marc Wadsworth" and carried placards in support of him.
One of them said that the rows about anti-Semitism – prejudice against Jewish people – in Labour had been "largely fabricated".
Mr Wadsworth, an activist and anti-racism campaigner, told reporters: "I am not guilty. If this hearing is based on the facts and it's fair, I will be exonerated."
The case is based around events at the launch of Shami Chakrabarti's report into anti-Semitism within Labour in 2016.
The hearing is expected to last most of the day with a decision most likely on Thursday.
Ms Berger, who was accompanied by Louise Ellman, Jess Phillips, Anna Turley and Margaret Hodge among others, said what they wanted was "strong action" not just "warm words".
"It has been 22 months now. There is a lot of talk about due process but actually process has happened.
"Everyone, including all my colleagues who have joined in solidarity with Ruth Smeeth, will be looking for some strong action. I anticipate that is what will happen."
One of the MPs who arrived with Ms Smeeth, described the scenes outside as "appalling".
The outcome of the hearing, which will decide whether Mr Wadsworth should be expelled from the party, is not expected until Thursday at the earliest.