The latest data on Population of capital cities and cities of 100,000 and more inhabitants is availabe in PDF and Excel format as table 8 of the UN Demographic Yearbook. The table can be accessed through the following link:

http://unstats.un.org/unsd/demographic/products/dyb/dyb2.htm

For data pertaining to previous years, please consult UNData (http://data.un.org/) and search for “city population”.