The Palestine Solidarity Campaign has pledged to do everything possible to block the government's proposed change to universal jurisdiction legislation.
Speaking at a rally to mark the second anniversary of Israel's Operation Cast Lead, PSC campaigns director Sarah Colborne urged supporters to pressure their MPs and recruit friends to the campaign.
She said: "We want to send a very clear message. Don't fête war criminals. We will find other routes to bring you to justice. We will not stop until we bring those responsible for war crimes to justice."
Around 250 people attended the event in central London on Tuesday evening.
There was loud applause when PSC chairman Hugh Lanning highlighted the report by the Reut Institute, the Tel Aviv-based policy group, which declared London to be a hub of anti-Israel action.
Veteran campaigner Tony Benn, 85, said he did not believe in a two-state solution to bring peace to the Middle East: "The answer is a single state where Jews and Palestinians live together. To do that we have to dissolve Zionism."
Hip-hop artist Lowkey, whose real name is Kareem Dennis, said: "Today the state of Israel is every bit the terrorist state for which the Irgun and Stern Gang killed to bring into existence.
"Zionism isn't about self-determination, it's about colonialism, imperialism and ethnic cleansing. Never let people make you feel ashamed for being an anti-Zionist."
Jody McIntyre, the activist who accused police of pulling him from his wheelchair during student protests in London last month, attacked the British government's support of Israel.
"Our struggle is here in the streets of London, in Birmingham, in Manchester. This government does not represent us. We do not want to send one more weapon to an apartheid state. We refuse to continue to play bankroller to the oppressor."
Two members of the anti-Zionist Neturei Karta sect also attended the meeting.
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