Pro-Palestinian protesters interrupted a speech by shadow foreign secretary David Lammy, with cries of "will you condemn the genocide" and "how many more children need to die".
The Labour Tottenham MP was giving a speech to the Fabian Society on Saturday to outline the party's foreign policy if it wins the next general election.
But he was interrupted by an activist draped in a Palestinian flag, who stormed the stage and shouted criticism of Labour's policy on the Israel-Hamas war.
Once the protester was ushered out of the Guildhall in central London, a second person stood up and shouted: "Call yourself a socialist?"
Mr Lammy laughed, and replied: "I was born in Tottenham, don't worry... we all want to see a sustainable ceasefire in Gaza."
At least two more protesters interrupted his speech, with further outbursts of "you should be ashamed of yourself" and "blood on your hands".
Mr Lammy replied each time with the phrase: "Change through power not process."
Labour leader Sir Keir Starmer faced criticism at the beginning of the war for saying "Israel has the right to defend itself", without a similar statement in support of the people of Gaza.
He has since hardened his position, particularly with regards to Israeli Prime Minister Benjamin Netanyahu, who this week suggested he no longer supports a two-state solution when the current conflict ends.
Sir Keir said the comments were "unacceptable", with Mr Lammy echoing: "I have to say, I think Netanyahu's words were unacceptable.
"Of course, the Palestinian people deserve a state and if they don't, the consequence of that is either one state in which Benjamin Netanyahu would have to explain how Palestinians and Israelis live side-by-side with equal rights, or no state, in which what he's really saying is occupation and siege continues."