Zentral-Dhaka, vielleicht der bevölkerungsreichste 3-Kilometer-Kreis der Welt.

Almost as many as in the country of Mongolia

Now do Dakar

Surat, India has 4M in 3 km radius (check same tool) with Dhaka being the second densest. for 5km radius, Dhaka is #1 by far (7.4M vs #2 Karachi 5.6M).

I guess Surat is really concentrated in that 3 km radius circle because just going out of it the population drops fast, whereas Dhaka has consistent remarkable density over a 5km radius.

For 10km / 15km radius, Karachi was the highest one I found (14,4 M vs Dhaka 14.1 M in 10km), but for higher radius Dhaka was once again the densest.

For 100km radius circle, the most populous one is 70.4M centered at Dhaka (even though the majority of it is rural), but Shanghai metro and Pearl River Delta comes close as well (68M+).

Can you share link of this tool