I was breifly detained on the street in London for an anti-terrorism search shortly after the tube bombings. I'm early 20s, male, was wearing a backpack, and happened to have some facial hair at the time. Two MOD officers made a beeline for me shortly after I exited a tube station and asked if they could search my bag. Being the rabble-rousing colonial that I am, I answered in the negative. They asked a couple more times, with me giving the same answer. After the third attempt, they quit being polite and admitted that they were telling, not asking. Went thru my bag and pockets, ran me thru the computer, and made me produce ID. They were quite excited, as it was the first time they had seen an American driver's license. They gave me a receipt and I was on my way within about 10 minutes. I honestly think I got picked by CCTV while I was still in the tube as quick as they were on me. The rest of my trip, I left the bag at home and made sure I read a newspaper on the train so I blended in a little better. Definately made me appreciate the 4th Amendment a little more!