What is the official source of data on the web about US cities - population, age, sex, income etc ?

I need to gather all sorts of information about US cities (demographic, geographic etc) what are official/best resources for this? Is there a website where I can download this info? Ideally, it should be possible to download the data in excel format or similar.

Public Comments

1. The U.S. Census Bureau identifies and tabulates data for the urban and rural populations and their associated areas for the presentation and comparison of census statistical data. Check out http://www.census.gov/ In order to download data about U.S. cities, you might want to try "American Fact Finder" at http://factfinder.census.gov/home/saff/main.html?_lang=en Also, check out the American Community Survey available at http://factfinder.census.gov/servlet/DatasetMainPageServlet?_program=ACS&_submenuId=datasets_2&_lang=en