The shadow chancellor has said the decision to bomb Syria is a "shameful step backwards" and warned it could foster another generation of militants determined to commit a Paris-style terror attack.
John McDonnell said the air strikes launched this week would result in the loss of lives and could make Britain less safe.
And he reiterated his comments about Hilary Benn's widely acclaimed speech in favour of the strikes against Islamic State (IS) - warning that the finest oratory can lead to the greatest mistakes.
Addressing The People's Assembly conference in central London, Mr McDonnell said: "I have to say this week was a disgraceful, shameful step backwards. And I find it appalling.
"I know some people found some of the speeches in the great parliamentary tradition and all the rest of it.
"But as I commented afterwards, some of the greatest oratory in the past has led us towards our greatest mistakes.
"I think what we will see from the decision this week is not just loss of life, but I also think it will embed, for another generation unfortunately, some of the issues and problems that we saw in Paris.
"It is a tragedy, an absolute tragedy."
He said a political solution should have been pursued rather than a bombing campaign.
He also called for the Government to stand up at "long last" to support the refugees fleeing conflict in Syria.
He said: "We are standing back with minimal commitment, and as a result people are dying in the Mediterranean and people are enduring appalling conditions in Syrian camps."
The left-winger said the new Labour leadership had set out to change Britain's foreign policy from military aggression to conflict resolution.
Labour leader Jeremy Corbyn bowed to pressure and allowed his party MPs a free vote on whether to extend air strikes against IS into Syria.
More than a quarter of Labour's MPs voted with the Conservative Government for the bombing campaign.
Among them was shadow foreign secretary Mr Benn, whose emotional speech on the matter was widely seen as the finest speech of the debate.
Mr McDonnell also attacked the new Trade Union Bill, which proposes higher thresholds for ballots.
And he backed comments made by Unite general secretary Len McCluskey, who has said workers have a right to break the law and strike if the Bill is passed.
Mr McDonnell accused the Government of wanting to "take the trade unions out once and for all".
He said: "What we say on that, yes we will oppose the Bill as it goes through Parliament. We will seek to amend it as best we can both in the Commons and the Lords.
"But if it's implemented against any single trade unionist or trade union, the resistance has to be there in terms of overall solidarity.
"And as Len McCluskey has said, if that means we have to resist an illegitimate law and we have to step outside the law to do that, so be it.
"That is the only way we secured basic human rights in this country, because we had to campaign against illegitimate laws.
"This is an attack on basic human rights."