On Urban Density

San Francisco 6,898 people per km2
New York City 10,725 people per km2
Tokyo 6,000 people per km2
London 5,354 people per km2
Paris 22,000 people per km2

From the Wikipedia page on Paris:

Paris is one of the most densely populated cities in the world. Its density, excluding the outlying woodland parks of Boulogne and Vincennes, was 24,448 inhabitants per square kilometre (63,320/sq mi) in the 1999 official census, which could be compared only with some Asian megapolises and the New York City borough of Manhattan. Even including the two woodland areas, its population density was 20,169/km2 (52,240/sq mi)[.]

Here is a list of US cities by density.