As it was still pretty early, it took me a while to connect this with a half-heard news item about President Obama addressing the Houses of Parliament.
I filtered up Whitehall to Trafalgar Square, where a police motorcyclist was holding up the traffic.
Pretty soon several other police bikes whizzed past in the opposite direction, and I took this to mean that stuff would happen soon.
Some moments later, posh cars full of scary dudes in black suits and sunglasses began to roll by, then vans with gentlemen in all-black combat gear.
There were two long black cars with dark windows and US flags on the bonnet, one of which, I assume, contained Mr Prez. And that's how I passed within a few metres of the most powerful man on Earth.
Several minibuses with 'DELEGATE' notices inside the windscreen brought up the rear, and I was allowed to proceed to Charing Cross Road and thence to work five minutes late.