It was a heavy blow to an already reeling government. A hundred thousand marchers thronging the streets of London plus thousands more in Glasgow and Belfast.
Banners as far as the eye could see - unions, Labour Party branches, the Communist Party, CND, Stop the War.
Home-made placards demanding everything from nuclear disarmament to George Osborne's arrest for fare-dodging.
And a unanimous call for a general strike to stop the cuts and start building a better world.
It's becoming increasingly clear that we may need action on that scale to defeat a coalition hell-bent on destroying and selling off as much of our welfare state and our public services as possible before they're finally booted out.
But slogans, banners and stirring speeches are one thing. Action is another. The challenge now is to turn Saturday's inspiring display of unity into a massive popular force that can bring down the rotten Con-Dem government.
And that is going to take work. We need to build support everywhere we can possibly get it. From the top to bottom of the trade union movement - the TUC, trades councils, local branches.
In public and private-sector workplaces across the country among the workers facing job cuts, pay freezes, spiralling rents and soaring food and fuel prices. In universities and colleges among the young people who are set to become a "lost generation" weighed down by joblessness and massive debt.
There is little stomach for the Con-Dem cuts - and there will be still less once they really start to hit home next year.
But people won't fight back unless they know there is an alternative worth fighting for.
Although it was welcome to see Ed Miliband address crowds in Hyde Park on Saturday, the Labour leader is still clinging to the same dismal and uninspiring "too far, too fast" line on austerity.
Britain needs better than that. Miliband must do better than that.
Our campaign should be built on the bedrock of the People's Charter - six simple demands ranging from jobs and decent homes to public ownership of banks and basic amenities.
It's a clear vision for a fairer, better society. A message of hope in defiance of the Con-Dems' doom and despair. And a manifesto broad enough for the entire left to rally around.
We can sign up worker after worker, branch after branch, union after union to the charter and speak with one voice to support its demands.
But we need a voice inside Parliament too. And Labour, the party of the workers, is failing to speak up for us.
So part of the big campaign - again, right from the grass roots to the top of the labour movement - must be directed at Labour.
Miliband must be made to realise that a time when Britain's workers are facing class war on an unprecedented scale, this is his and Labour's last chance.
If Labour will not stand and fight now for the people it speaks for and the people who fund it and campaign for it, then it never will.
The party's members and affiliated unions need to issue a simple ultimatum to Miliband.
Back the People's Charter, defend our interests, stop the Tory cuts. Or risk the labour movement abandoning your party to the neoliberals and setting up a new one that is prepared to fight for a better Britain.
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