The Washington Post
http://www.washingtonpost.com/wp-dyn/content/article/2009/03/21/AR2009032101368_pf.html
says:

"Organizers estimated yesterday's crowd at 10,000, but Arlington County police said the crowd was between 2,500 and 3,000."

But a post on the UFPJ discussion list says:

I was at the demonstration in DC and I think it was about 10,000. It was certainly more than 2-3000. I found it unexpectedly inspiring.
The most touching talk was by a black woman who lost her son in Iraq. Her message was simple - the wars must end. They are wrong whether we have a white president or a black one.

The young were as usual the most imaginative and aggressive. I especially liked: "Barack Obama, USA; How many kids did you kill today." Or "We support the war resisters; they're our brothers, they're our sisters." Etc. Code Pink also did a great job, actually organizing for the demo and showing up in force. (Some like PDA gave a paper endorsement, a good step forward for them but did nothing to organize. And then there is the UFPJ leadership....)

The demonstrations serve an important function. They let people know that there are people out there who actively oppose the war. Most people form their opinions on the basis of a kind of multiple choice test - and we are putting another choice on the table. If people do not see others with the same opinion out in public, they lose confidence in their own opinion to some degree.

So this is an historic demonstration - precisely because of what that Black mom said. If anyone has a better plan, let me know. But as Ralph Nader says, "Surrender is not an option."
jw
p.s. I have my bumper sticker now, "Don't blame me, I voted for Ralph."

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Photos on John Ks Facebook, who thought the demonstration was between 7000 and 10000 people.