Here are translations of excerpts from
Rav Shach's letters (printed in Mechtavim v'Ma'amorim volume 3, pages
100-102).
"...His followers have reached the lowest level, to
speak things that are forbidden to say or even to hear; to designate
the building they've built for their leader with the name "Third
Temple". And they've shamelessly done things that our
fathers would never have even imagined.
"For this, I see it as my responsibility
to warn (for the many people who can't properly distinguish between
right and wrong) how we should separate from them and their multitudes,
and HaShem should send us quickly Moshiach Tzidkainu, and not test us
with false messiahs"
"I have heard that in the yeshiva there
are many who have been distracted by the study of books of the
chassidus of Chabad. This isn't according to the tradition of our
teachers and it is better to learn Sha'arei Teshuva l'Rebbainu Yona,
Mesilas Yesharim, Nefesh Hachaim and similar mussar seforim. For
more than this there is no need and [on the contrary] it can cause more
harm than good. But especially the sichos of Chabad, that today
contain things that are forbidden to hear (including) many words of
kefira therefore it is necessary to warn about this - but to do it
intelligently."