Are both sentences OK? Or, what do you say?
The fans have crammed the stadium.
The fans have filled the stadium up.

Question

Are both sentences OK? Or, what do you say?
The fans have crammed the stadium.
 The fans have filled the stadium up.

Answer

I think "The stadium is crammed with fans" is more acceptable.
I agree with Hugo: The fans have filled up the stadium" is a better option.

Hope this helps :)

Answer

You might hear someone say, "The fans are crammed into the stadium."
That is more casual language.

"The fans have filled the stadium." or "The fans have filled up the stadium."
These are a little more formal.

Answer

Crammed the stadium is not something that I've ever heard used as a native speaker.

I would suggest "the fans have filled up the stadium".