What If We Nuke a City?

In 1945 two atomic bombs were dropped on Hiroshima and Nagasaki, killing 50,000 people each and unleashing 20 kilotons of TNT. The bombs dropped on Japan decimated entire cities, but the Tsar Bomba is 1,570 times more powerful. Since WW2 cities have only become denser and more populated, so it’s worth asking, what would an explosion of that size be due to some of today’s largest cities?