This book is by Terry Carr. It has somehow ended up under Philippa Carr booklist.
Millennia in the future, Earth has become a backwater planet, ignored by others in the galaxy. Its one jewel is Cirque - the city on the Abyss, a city of love and harmony, with inspiring religious rites.
But in the Abyss there lives the Beast, formed from the castoff hates of the Cirquians: a beast whose body is refuse, whose mind is black as sin. Feeble weapons are no match for the Beast.
And now, after centuries, it's climbing out of the Abyss to claim its own...