Out of the world's population, what percent live in Urban areas?  What are the 
largest 20 cities in the world in order?

It seems about half of the world's population lives in Urban areas. 

The Population Reference Bureau located in Washington D.C. has an interesting 
article on its website titled 'Human Population: Fundamentals of Growth 
Patterns of World Urbanization'. This article contains reports from the year 
2000. According to their website, "..about 47 percent of the world's population 
lived in urban areas, which is about 2.8 billion..". It is quite an increase 
considering the fact that "..in 1800, only 3 percent of the world's population 
lived in urban areas.."

The site goes on to say that "..It is expected that 60 percent of the world 
population will be urban by 2030, and that most urban growth will occur in less 
developed countries.." 

The above information was taken from Population Reference Bureau website:

http://www.prb.org/Content/NavigationMenu/PRB/Educators/Human_Population/Urbaniz
ation2/Patterns_of_World_Urbanization1.htm

Similar data is found on the United Nations Environment Program (UNEP) site. 
UNEP's Global Environment Outlook (GEO) has surveyed Growth of Urban 
Populations. Their website contains an article which says "..Almost 3000 
million people, about half of the world's population, live in urban areas and 
about 160 000 more join them every day.."

The above information was taken from UNEP website: 

http://www.unep.org/geo2000/english/0049.htm

For Urban / Rural division breakup per country please visit:  

http://www.geohive.com/global/pop_urban.php

According to this site, 
World Population:
Urban  2,923,182,000  (47.7%)
Rural  3,210,954,000 
Total  6,134,136,000 

In addition to the above, the following sites have a lot of useful global 
statistics on population.

http://www.citypopulation.de/ 
and
http://www.world-gazetteer.com/home.htm

-------------------------------------------

There are different ways to define the boundaries of a city. The city 
population website: 

http://www.citypopulation.de/faq.html

defines an "agglomeration" as "..include a central city and neighbouring 
communities linked to it by continous built-up areas or by many commuters.."

According to this definition, The Principal Agglomerations of the World (as of
14.10.2001) are: 

(Taken from: http://www.citypopulation.de/Country.html?E+World)

1. Tōkyō Japan, incl. Yokohama, Kawasaki 
2. New York, USA incl. Newark, Paterson 
3. Sŏul (Seoul), South Korea incl. Inchon, Songnam 
4. Ciudad de México, Mexico incl. Nezahualcóyotl, Ecatepec, Naucalpan 
5. São Paulo, Brazil incl. Guarulhos 
6. Ōsaka, Japan incl. Kobe, Kyoto 
7. Los Angeles, USA incl. Riverside, Anaheim 
8. Mumbai (Bombay), India incl. Kalyan, Thane, Ulhasnagar 
9. Al-Qāhirah [Cairo, Kairo] Egypt incl. Al-Jizah, Shubra al-Khaymah 
10. Jakarta, Indonesia  
11. Buenos Aires, Argentina incl. San Justo, La Plata 
12. Kolkata (Calcutta), India incl. Haora 
13. Moskva [Moskau], Russia  
14. Delhi, India  
15. Manila, Philippines   
16. Shanghai, China  
17. London, Great Britain and Northern Ireland   
18. Rio de Janeiro, Brazil incl. Nova Iguaçu, São Gonçalo 
19. Karāchi (incl. div. cantonments), Pakistan 
20. İstanbul, Turkey

The world gazetter lists both the top 20 metropolitan areas as well as the top 
20 cities at this URL (see below), where the "city" includes only the urban 
center (not including surrounding areas).

URL: http://www.world-gazetteer.com/home.htm

Place         Population(1000s) Continent   Country 

Buenos Aires  12955.3         The Americas  Argentina 
Mumbai        12147.1         Asia          India 
Seoul         11153.2         Asia          Korea (South) 
Jakarta       10810.4         Asia          Indonesia 
Karâchi       10272.5         Asia          Pakistan 
Manila        10133.2         Asia          Philippines 
São Paulo     10057.7         The Americas  Brazil 
Dilli         10009.2         Asia          India 
Istanbul       9216.4         Europe/Asia   Turkey 
Shanghai       9031.2         Asia          China 
México         8656.8         The Americas  Mexico 
Dhâka          8539.5         Asia          Bangladesh 
Moskva         8376.0         Europe        Russia 
Tôkyô          8180.5         Asia          Japan 
New York       8038.6         The Americas  United States of America 
Lagos          8029.2         Africa        Nigeria 
al-Qahira      7764.7         Africa        Egypt 
Tehrân         7722.9         Asia          Iran 
Lima           7603.5         The Americas  Peru 
London         7393.8         Europe        United Kingdom 


Search terms used in Google:
World population live urban
largest cities population

Hope this information was useful!

//--Comment--
  I feel like Var's asnwer is very precise and well done and probably some 
information contained on this comment will be redundant.  However I started 
the research before the question became locked, and at the time of submitting 
it was Impossible for me as it was locked.  I spend a considerably amount of 
time, so rather that throwing the job to the bin, here are the results of my 
research.  I hope it adds valuablwe information to Var's answer :) --//

Hello!
 In a German website called 'citypopulation' there is a FAQ containing this 
exact question. <http://www.citypopulation.de/faq.html>.  Answering to 'What 
are the n largest cities of the world?' it is said that taking into account 
the arbitrary city boundaries, it is not really reasonable to compare their 
populations and that any given list would hardly be accurate.  However, in 
that same website there is a map showing the most populated agglomerations of 
the world and it is suggested to visit the 'Global Population Project' 
and 'Global Statistics'.  Therefore I recommend you to visit the above 
mentioned webpage if your are looking for further detail.
  Nevertheless, I was able to find several listings which prooves that these 
lists can hardly be accurate as they vary greatly one from another. 

A list of the 50 more populated cities, containing interesting links at:
One World - Nations Online
<http://www.nationsonline.org/oneworld/bigcities.htm>

A list of the 10 largest cities in the world according to to the book "The Top 
10 of Everything" by Russell Ash is available at ABOUT:
Geogrpahy - About
<http://geography.about.com/library/faq/blqzdensecities.htm>


A list containing the 10 largest cities in 1975, together with other dates and 
estimations for the future at:
Largest cities - 1975
<http://www.sru.edu/depts/artsci/ges/discover/d-6-9a.htm> 


   About your question on what percent live in urban areas, according to the 
Population Reference Bureau, about 47 percent of the world's population lived 
in urban areas in the year 2000.  That is about 2.8 billion people. This page 
can be reached at the following address:

Patterns of World Urbanization - Population Reference Bureau
<http://www.prb.org/Content/NavigationMenu/PRB/Educators/Human_Population/Urban
ization2/Patterns_of_World_Urbanization1.htm>


I hope this has helped! :)

--Toksuri

Search Terms:
- largest cities in the world
- most populated cities
- world population urban areas