Richard Rogers, life peer and long-standing Labour supporter, was one of 20 architects who put their names to anti-war newspaper advertisements before the march.
Rogers set out his case in an interview with the Mail on Sunday. He said he felt so strongly that the mobilisation for war was premature, misguided and morally wrong that he had no choice but to join the demonstration.
He said: "I hope the government will take into account the weight of public opinion evident in the numbers who turned out, united in their opposition and angry that the prime minister seemed to have so little regard for the democratic process."
Edward Jones, co-founder of Jeremy Dixon.Edward Jones, put his name to the advertisement and went on the march. He told Building: "The march on Saturday was a massive public demonstration that the government, if it represents the people, cannot ignore."
Will Alsop of Alsop Architects, another signatory, was unable to go on the march, but supported it.
He said: "We need to keep the pressure up, although I suspect a date has already been set to go into Iraq."
Alsop's view was backed by John Miller, who also supported the Rogers initiative. He said: "To be honest, I think they got their diaries out in November and decided that the beginning of March was a good date to go to war."
Ivan Harbour, a partner in Richard Rogers' practice, said: "In the interests of humanity, war should be avoided if at all possible. With regards to the industry, war will always have an impact on business."
Robin Nicholson, a senior director at Edward Cullinan architects, also made his anti-war views clear. He said: "I think it's important for people to express their views. I cannot believe the government would have failed to notice the march." The founder of the firm, Edward Cullinan, signed the advertisement opposing war.
I think it was important for people to express their views. I cannot believe the government could have failed to ignore the march
The last time I marched was in protest at Vietnam. I think it was great that so many people were motivated to get off their arses and protest
It was a great success in terms of numbers, demonstrating that the attitude of Tony Blair does not represent that of a huge number of people
This week’s web poll asked:Should the UK take part in a war against Iraq?
Yes, with no UN mandate if necessary - 16%
Yes, but only if backed by the UN - 34%
No - 50%