Which is correct - 'Hundreds of thousands of people' or 'hundreds and thousands of people'?

Hundreds of thousands of people! Answer hundreds and thousands of people Answer As you can see, both alternatives are used, and probably they are equally correct. I would tend to use "of". You may have heard people say "tens of thousands", and this is correct usage as well. The idea is that "tens" and "hundreds" are ways to give a rough idea of scale.

I could say "There are thousands of people in New York City." "Thousands" is not entirely wrong; there are in fact many, many thousands of people there (a thousand thousands for each million). But thousands isn't the best estimate of magnitude. If I say "thousands", I mean a few thousand, maybe even a few more than 10. But if I say "tens of thousands", I mean, perhaps, 30, 40, 50, 60 thousand.

Thousands, around the order of magnitude "10". Hundreds and thousands, to my ear, does not convey the same general magnitude of size. Of course, there is nothing even remotely accurate about these estimates, but they help us grasp what the speaker is attempting to convey.

Hundreds and thousands means multiples of hundreds (100s) and/or thousands (1000s). But hundreds of thousands (100000s) means multiples of a much bigger number indeed. So both are correct expressions but they convey the idea of quite different sizes of number.