Dies is a demonstrative pronoun with the declination table to be found here. It always refers to an element of a set.
„Dieses Buch gehört Peter.“ refers to one (Peter's) element of the set of books.
In contrast du posessive pronouns, they cannot refer to something established later on.
„Sein Geschenk hat Peter nicht gefallen.“ works perfectly fine.
„Dieses Buch gehört Peter.“ works, because the set is established along.
„Dieses ist Peters Buch.“ doesn't work on its own, because the set is established later on. It can, however, work if the set of books was established in a sentence before or is conclusively established along by pointing.
dies can refer to some unknown set (or the class of everything) as well. In that case, it isn't declinated.
In you case,
dies refers to some unknown set. So it should be
Dies sind Zeitungen.
Depending on context,
Diese sind Zeitungen. might work as well, for example, if a shopowner shows you his goods (
Waren) and names them.