Did you attend one of the protest marches on Saturday?
Which city/country?
Did you participate or observe, or both?
What were your impressions?
I went to the Philadelphia march just to see what it was like and take pictures.
It looked huge, but was reported to be about 10,000 people. It was orderly. You could identify (by banner, sign, etc) various subgroups within the crowd: students, Quakers, etc.
Try these threads:
http://boards.straightdope.com/sdmb/showthread.php?threadid=163342
http://boards.straightdope.com/sdmb/showthread.php?threadid=163340
Yes, as did brothers, parents, tiny nieces (who must have been a bit amazed to see SO many people! 
Glasgow, Scotland.
Participate
Impressions? That it was amusing that Tony Blair (who was in Glasgow to address the Labour Party Spring Conference in the Exhibition & Conference Centre, outside which the main demo was held) altered the timing of his speech to avoid the protest. Next tiem I hear him suggest that he is unworried about public disapproval of his stance, I can have a good laugh (albeit it sort of hollow laughter.)
Numbers here - possibly approx 90.000. And it is widely agreed, that, like many other marches elsewhere, many protesters were people who had never previously attended such a demonstration. Oh, and again like other marches, it was peaceful and trouble-free. (I believe there were 4 arrests.)