of whom

You could also say:

Twenty people were arrested during the demonstration, FOUR OF WHOM were charged with obstruction.

Emily__Hi, could you tell me whether both sentences are correct?

Twenty people were arrested during the demonstration, of whom four were charged with obstruction.

Twenty people were arrested during the demonstration, four of them were charged with obstruction.

Thank you.

Both are correct, but the first one would seldom be said

The first sentence "of whom......" is a nonidentify clause. That modfies the sentence.

I am not sure the second is correct. I would use a semicolon or add a connector.

Twenty people were arrested during the demonstration; four of them were charged with obstruction.

Twenty people were arrested during the demonstration only four of them were charged with obstruction.

Twenty people were arrested during the demonstration, four of them were charged with obstruction.

I prefer a semi colon before "four".