They were cleaning up the mess in London yesterday — replacing the windows at the Ritz, fixing the wooden façade at Fortnum & Mason and attending to a vandalized Trafalgar Square in the wake of Saturday’s protests — and riots — by trade unionists and anarchists.

Think it can’t happen here? Think again.

The British press reports that up to half a million people took part in the demonstrations, 200 were arrested and more than 160 injured — including 84 police officers, 11 of whom had to be hospitalized.

Why? Because Prime Minister David Cameron announced a $130 billion cut in public spending and this is how the infantile left reacts when its I’ve-got-mine-Jack gravy train is threatened.

The weekend demonstration was led by Britain’s powerful Trades Union Congress, whose leader, Brendan Barber, has been warning for more than a year that “industrial unrest” would greet any Tory government that dared attack Britain’s sky-high social spending.

Addressing the gathering in historic Hyde Park was Ed Miliband, the Labor Party’s new hard-left leader (his father was a prominent Marxist), who likened the protesters to the suffragettes, the US civil-rights movement and the fight against apartheid in South Africa. Which ought to tell Britons all they need to know about their prime-minister-in-waiting.

In the aftermath of the all-too-predictable violence — in Trafalgar Square, the words “fight back” and “Tory scum” were scrawled on the bronze lions and red paint was splashed on the 2012 Olympics countdown clock — there was the usual tut-tuting.

Barber, the union chief, said he “bitterly regretted” the violence, then added, “I don’t think the activities of a few hundred people should take the focus away from the hundreds of thousands of people who have sent a powerful message to the government today.” Right. The government had better get the message — or else.

Expect more of this in the coming months and years, as countries finally face up to the fact that the money has run out and that tough choices must be made — not just nickel-and-dime decisions about which programs to cut but the harder issues of whether certain programs should even exist.

We’ve already seen it, on a smaller scale, in Wisconsin, during the recent battle over Gov. Scott Walker’s plan to rein in the public-employee unions. In Madison, the capitol was occupied by hordes of protesters and the lives of some Republican state legislators were threatened.

This is no way to run a democracy. Peaceful protests are one thing, but massed force and ominous warnings about dire consequences are another. Throw into the mix the free-floating anarchists who routinely show up at such events — most recently at the G-20 summit last year in Toronto — and you have a prescription for serious trouble.

Yet, all too often, any attempt to open a civil discussion about the future is met with the same dreary charges that “hateful” conservatives want to kill old people and steal candy from babies.

For some on the left, too much is never enough — because, by definition, it can’t be. They operate on a modified version of the old Brezhnev Doctrine, which stated that once a country went communist, it could never go back: Once a government program is in place, it can never be cut or rescinded, only fattened. It doesn’t even matter whether it’s effective. The self-interested and the self-deluded have too much to lose to give up the fantasy of the perfect nanny state.

The problem is, we no longer have that luxury. “Progress” need not be the exclusive province of the so-called “progressives” — and thuggery should have no place in our political system.

People of good will on both sides need to stand strong against intimidation and violence and insist that such questions be settled at the ballot box — and that the results be respected. In any contest, there gets to be two teams on the field, not just one, and the game can only be played if both sides follow the rules.

When one doesn’t — well, just take a look at Trafalgar Square, then brace yourselves.

Michael Walsh, a former associate editor of Time, is the author (writing as David Kahane) of “Rules for Radical Conservatives.”