Gleaned from various presses, source of estimate where known or specified by news source:

New York: 375,000-500,000 (sez organizers, police did not offer an estimate, stationary rally stretched 2 blocks wide by 20 blocks long)
Manhattan: 1,000 -- counter protest in support of war
Wausau, WI: 200 -- counter protest in suport of war
Los Angeles: 30,000 (police), 100,000 (organizers)
Denver: 300 -- counter protest in support of war
Montreal: "over 20,000"
Toronto: "over 15,000"
Mexico City: 10,000
Puerto Rico: 900

Brazil: 1,500 (police)

Sydney: 250,000 (authorities), 500,000 (organizers)
Canberra: "tens of thousands" (Reuters), 16,000
Brisbane: 20,000
Perth: 10,000
Newcastle: 15,000
Melbourne: 150,000

London: 750,000 - 2 million
Glasgow: 50,000-60,000
Dublin: 80,000 (police)

Amsterdam: 70,000
Oslo: 60,000 (police)
Brussels: 50,000
Stockholm: 35,000
Bern: 40,000
Copenhagen: 25,000
Vienna: 15,000
Rome: "650,000" (authorities), "1 million" (police), 3 million (organizers)
Kiev: 2,000
Athens: "several thousand"

Berlin: 300,000-500,000 (police)
Brussels: 50,000 (police)
Germany: (multiple cities incl. Berlin and Brussels): 500,000

Madrid: 660,000 (police), 2 million (organizers)
Barcelona: "over 500,000"
Seville: 60,000
Spain (55 cities incl. Madrid, Seville and Barcelona): 5 million (organizers)

Paris: 100,000
Toulouse: 10,000
France (multiple cities incl. Paris and Toulouse): 300,000

Cape Town: 5,000
Johannesburg: 4,000

Calcutta: 10,000
Tokyo: 5,000
Karachi: 300
Jakarta: 500 (protest last weekend was 7,000)
Dhaka: 1,000-2,000
Kaula Lumpur: 1,500

Cyprus: 500
Tel Aviv: 2,000, mixed Israeli and Palestinian
Damascus: 200,000
Baghdad: "tens of thousands" - not a peace protest, but a militia demonstration

Misc reports of marches in Indonesia, Malaysia, Thailand, India, East Timor, Pakistan, Kashmir, Moscow, Taipei, Hong Kong, Kazakhstan, Singapore, South Korea, Mostar, Budapest, Lyon, Marseilles, Sofia, Reykjavik, Antarctica, Stuttgart, Thessaloniki, Warsaw, Colorado Springs, CO, Yakima, WA, Augusta, ME, Chicago, IL, Philadephia, PA, Miami, FL, Seattle, WA, Knoxville, TN, Albany, NY


This just in from SF Chronicle: "more than 100,000" Same report of crowd size from MSNBC. Police say 150,000. Organizers say 250,000.