From the ANSWER website:

Tens of thousands march on the Pentagon
50,000 march in Los Angeles, 40,000 in San Francisco
Le Monde, ran a significant article under the headline, "More than 50,000 People Protest Against the War in Iraq," about the March on the Pentagon

Quite a difference from what is reported elsewhere:

The Guardian:

   10,000 to 20,000 anti-war demonstrators marched, with a smaller but still sizable number of counterprotesters also out in force. An hour into the three-hour Pentagon rally, with the temperature near freezing, protesters had peeled away to a point where fewer than 1,000 were left.


The Washington Post
Veterans, Others Denounce Marchers Counter-Demonstrators Number in Thousands

Several thousand vets, some of whom came by bus from New Jersey, car caravans from California or flights from Seattle or Michigan, lined the route from the bridge and down 23rd Street, waving signs such as "War There Or War Here." Their lines snaked around the corner and down several blocks of Constitution Avenue in what organizers called the largest gathering of pro-administration counter-demonstrators since the war began four years ago.


API
About 200 police officers were on scene in Hollywood for security, said Los Angeles police Sergeant D. Costi.
Authorities estimated attendance at 5,000 to 6,000 people, said Officer Norma Eisenman.

War protest draws 3,000 in march to S.F. City Hall
Associated Press

SAN FRANCISCO - Thousands of marchers, angry but energized by a sense of growing support for their cause, closed down a major downtown thoroughfare Sunday in a largely peaceful display of opposition to the Iraq war.

Protesters march against Iraq war
BBC

In Los Angeles, police estimated that up to 6,000 people demonstrated in anti-war rallies which included flag-draped coffins being carried through the streets of Hollywood.