These were not protests against America. Plenty of Americans on the London march made that clear, they and we were marching in opposition to the British and US foreign policy on Iraq.
It's possible (if you can think in terms that aren't merely black and white that is) to disagree with US foreign policy and yet still have a great deal of respect and admiration for the USA. The US administration is not synonymous with The United States of America, thank heavens. Our countries (UK and US) have much to commend them, but now and again, we get leaders whose standards and behaviour fall below a critical threshold.
I marched in protest at my Government's willingness to commit our forces 'in defence of the realm' to a conflict that I believe will herald the start of many crises, rather than putting an end to an existing one.
And there's no 'supposedly' about it. Even the metropolitan police put the London figures at between 750,000 and 1,000,000 on the march alone between 11:50 and 5pm. the march went on until after 7pm and many, many people went straight for hyde park without marching for the rally. It was certainly the largest single gathering of people in the history of the British Isles (c:a 2000 years of it).