I was in Switzerland, I remember the day well.
I was supposed to be going to train with my athletics club, but there was no-one there; it turns out there was a track meet on at another stadium which I had completely forgotten about.
So I hung around the track until about 6pm local time, and then headed home.
I got in around 7pm, so about 1pm US time, and my little brother raced up to me to tell me that two planes had crashed into the World Trade Center, and that the towers had falled down.
Now, I was 11 years old and had never actually heard of the "World Trade Center", I initially thought he was talking about a rather scuzzy business park next to Geneva Airport, so it was only really over the course of the rest of the evening that I began to realise the significance.
The Swiss, and French TV channels were initially speculating on the cause, as was everyone, but I think they quite quickly pointed towards Al Qaeda (although I remember that Japanese cult Aum Shirinkyo being an early suspect). There was a fair amount of speculation that George W Bush had been warned, but that was playground rumour, rather than anything that was on the news.
There was pretty broad support for the US going into Afghanistan as well. Which really marks out a contrast to the invasion of Iraq two years later.