Which church council or father of the church pronounced that the pagan
gods did not exist or, if they did, they were demons? I am aware that
a statement something like this is made at Paul I Corinthians 10.19-20
but I am looking for something later, probably earlier than Ambrose. I
would like exact references.

Hello pelagius,

There seems to be a running thread in the works of the early church
fathers to mock pagan beliefs.  The pagan gods were either made out to
be demons, or said not to exist.  It seemed to be a frequent tactic to
argue that the stories of the pagan gods showed that they were really
men and not deities.  I’ve found three excerpts, by writers from the
2nd, 3rd and 4th centuries AD to show this continuity of thought.  The
earliest I managed to find, by Clement of Alexandria, is somewhat
contradictory, in that in some places he argues that the pagan gods
can have no real existence, while in others he maintains that they are
demons.

These excerpts are taken from the full text archive of the early
fathers of the church at http://www.ccel.org/fathers2/  It is
searchable, but the search facility was not all that useful, so I
ended up trawling through quite a number of works to try and find some
references of the sort you require.  I hope that you find these three
satisfactory, but please request clarification if you think I can help
you further or if you feel that I did not fully cover your query.

1.  Clement of Alexandria (153-217 AD)  in Exhortation to the Heathen
sets out, as stated in the heading to Chapter II to show “The
Absurdity and Impiety of the Heathen Mysteries and Fables About the
Birth and Death of Their Gods.”

Here are some excerpts from a very long tirade:

“…held fast as they are in the double impiety, first in that they know
not God, not acknowledging as God Him who truly is; the other and
second is the error of regarding those who exist not, as existing and
calling those gods that have no real existence, or rather no existence
at all, who have nothing but a name.”
and later
“The most of what is told of your gods is fabled and invented; and
those things which are supposed to have taken place, are recorded of
vile men who lived licentious lives”
He then sets out to prove that the gods of the Greeks were in fact
humans. And then, speaking of the wickedness of these “gods” who
demanded sacrifices etc, he says:
“I can then readily demonstrate that man is better than these gods of
yours, who are but demons; and can show, for instance, that Cyrus and
Solon were superior to oracular Apollo.”
and later:
“How, then, can shades and demons be still reckoned gods, being in
reality unclean and impure spirits, acknowledged by all to be of an
earthly and watery nature, sinking downwards by their own weight, and
flitting about graves and tombs, about which they appear dimly, being
but shadowy phantasms? Such things are your gods-shades and shadows;
and to these add those maimed, wrinkled, squinting divinities the
Litae, daughters of Thersites rather than of Zeus. So that
Bion-wittily, as I think-says, How in reason could men pray Zeus for a
beautiful progeny,-a thing he could not obtain for himself?”
http://www.ccel.org/fathers2/ANF-02/anf02-50.htm#P2691_785581


2.  Cyprian of Carthage in his Treatise VI.: On The Vanity Of  Idols:
Showing That The Idols Are Not Gods, And That God Is One, And That
Through Christ Salvation Is Given To Believers, which was written in
247 AD says:
“These demons the poets also acknowledge, and Socrates declared that
he was instructed and ruled at the will of a demon; and thence the
Magi have a power either for mischief or for mockery, of whom,
however, the chief Hostanes both says that the form of the true God
cannot be seen, and declares that true angels stand round about His
throne. Wherein Plato also on the same principle concurs, and,
maintaining one God, calls the rest angels or demons. Moreover, Hermes
Trismegistus speaks of one God, and confesses that He is
incomprehensible, and beyond our estimation.”
The Treatises of 
http://www.ccel.org/fathers2/ANF-05/anf05-116.htm#P7351_2431968


3.   Lactantius (most active 303-311 AD) in The Divine Institutes Book
I. Of the False Worship of the Gods argued from a viewpoint of sexism
that the pagan gods could not exist:
“Of the two sexes the one is stronger, the other weaker. For the males
are more robust, the females more feeble. But a god is not liable to
feebleness; therefore there is no female sex. To this is added that
last conclusion of the former argument, that there are no gods, since
there are females also among the gods.”
http://www.ccel.org/fathers2/ANF-07/anf07-04.htm#P135_2252

tehuti-ga could not know my purpose but it happens that one part of
the information he supplied was particularly appropriate. This was
possible because he provided a quite wide spread.

I'd add this to the list as well, for your consideration:

"But the zeal of Theodosius was not wholly absorbed in the
establishment of uniformity among the professors of Christianity; he
was equally anxious to extinguish the expiring embers of Paganism.
About the year 390, he issued a law, in which he expressly states that
"it is our will and pleasure, that none of our subjects, whether
magistrates or private citizens, however exalted, or however humble
may be their rank and condition, shall presume, in any city or in any
place, to worship an inanimate idol, by the sacrifice of a guiltless
victim." [Theod. 1. 16. tit. 10. leg. 12.] The act of sacrificing, and
the practice of divination by the entrails of the victim, are declared
a crime of high treason against the state which can be expiated only
by the death of the guilty. The rites of Pagan superstition are
abolished, as highly injurious to the truth and honor of religion; and
luminaries, garlands, frankincense, and libations of wine are
enumerated and condemned."

[from:  http://www.wayoflife.org/articles/jones16.htm]

Hello pelagius,

I'm glad my answer was satisfactory and that I hit on something extra
that is useful to you!