Helicopters whirling overhead. Police demanding a QR code and proof of identity to pass. Metal fences barricading the streets. And an army brigade, marching by in formation, machine guns hoisted to their chests.

This is what it has been like getting to the front door of my apartment building for the past few evenings.

It’s a weird moment in Paris, on the eve of the Olympic Games and its unusual opening ceremony. The heart of the city has been transformed into a fortress, with metal barricades and checkpoints, subway stations shuttered, 45,000 police officers — about 10 times more than normal — on top of 10,000 soldiers, search dogs, bomb squads and tactical teams, including in helicopters.

The Seine’s banks and bridges, normally teeming with humanity, are suddenly empty. A strange, exquisite calm has descended.

The reason: After all its hand-wringing over a Plan B, Paris is proceeding with its risky and wonderful plan to float the opening ceremony — with the world’s best athletes — down the Seine, through the center of the ancient city and before more than 400,000 spectators packed into stands and leaning out of windows.

Even if Paris weren’t scarred by terrorist attacks, protecting all those people forms an obvious — and heavy — security challenge.