Clash of the Titans? It's not an old movie though, from 2010 (though it is a remake of a movie of the same name from 1981)
Opening narration (by the character Io):
Io: The oldest stories ever told are written in the stars. Stories of
time before man and gods, when Titans ruled the earth. The Titans were
powerful but their reign was ended by their own sons: Zeus, Poseidon,
and Hades. Zeus convinced his brother Hades to create a beast so
strong it could defeat their parents. And from his own flesh Hades
gave birth to an unspeakable horror... the Kraken. Zeus became king of
the heavens. Poseidon, king of the sea. And Hades, tricked by Zeus,
was left to rule the underworld in darkness and in misery. It was Zeus
who created man and man's prayers fed the gods' immortality. But in
time, mankind grew restless. They began to question the gods and,
finally, rise up against them. Into this world, a child was born. A
boy who would change everything.
Has a sequel, Wrath of the Titans, in which the Gods are continuing to lose their power, weakening and eventually becoming mortal (and many dying) because of the lack of human prayer.