I was at work, and back then I worked in a warehouse / logistics office. We had the radio on and, like others, we imagined it was the start of the third world war. There was a question of whether we should be going home to our families from someone, but we stayed on as at least that gave some normality. I think I went straight round to my parents when I clocked off.
Can't really compare it to much else. Similar in how it hits to Hillsborough or even the Erikson thing this last summer, but magnified several fold. A bit like the Manchester bombings but again, you knew it was on a world scale. The free world was being attacked and, regardless of the flaws of the US and capitalism, that affects us all.
You have to remember that at the time nobody knew how far it went, who was backing the attacks and if more were to come. Whilst the day and the memory undoubtedly hits Americans harder, it has changed the lives of every one of us. Which makes the recent events in Afghanistan even more appalling.