Not much. The Stoic philosophers approved of slavery the least, perhaps. They emphasized the common humanity of slaves and slaveholders, but did not advocate for the abolition of slavery itself, so far as I know. Euripides similarly demonstrated empathy for the plight of slaves, but still felt the institution was necessary.

Among Christian writers, Gregory of Nyssa, a fourth-century bishop, was quite critical of slavery, but he still did not call for its abolition.

“The fact remains, however, that slavery caused no one in antiquity a crisis of conscience or an agony of the soul as it did abolitionists in later history, and as it still does some modern historians today. For a thousand years and more slavery was not a problem in classical culture.”

— Keith Bradley, “Problem of Slavery in Classical Culture,” Classical Philology 92 p. 282.

For more on the above, see The Cambridge World History of Slavery, Volume 1.

I can't speak to the situation in Asia.

There certainly was a certain "pro-democratic" sentiment living in Athens during the time of the Peloponnesian War (5th century BC), as exemplified by the case of Phaedo of Elis, who was given his freedom by Socrates, and by the way the Spartants distrusted the Athenians during their Messenian Revolt (see here).

Messenia was the large region of the Peloponnesus bordering Spartan territory on the west, which the Spartans had conquered in the eighth and seventh centuries and whose formerly free inhabitants they had enslaved as helots to farm the land for the benefit of the Spartans.

...

The Spartans feared that the democratically inclined Athenian soldiers might decide to help the helots (who were fellow Greeks) escape from Spartan domination.

However, as far as I'm aware, even in Athens there was never a motion for constitutional abolition. In the 6th century BC, Solon did outlaw the practice of debt slavery, but other forms of slavery (war captive slavery etc.) remained.

In China, Wang Mang abolished slavery in 9 AD as part of his edict on land reform. The edict also included nationalizing and redistributing all privately owned land. This is documented in the Han Shu, a history of the Han dynasty. An excerpt from a translation by Burton Watson (from page 364 of Volume 1 of Sources of Chinese Tradition) is provided below:

Now at this time let the term be altered and the land throughout the empire be designated "king's fields" and slaves be called "private retainers." Neither land nor slaves are to be bought or sold.

However, Wang Mang's land reforms were ultimately unsuccessful. They were reversed three years later.

In Aristotle's Politics Book 1 Part 3 ((1253b)(20)) he writes:

Others affirm that the rule of a master over slaves is contrary to nature, and that the distinction between slave and freeman exists by law only, and not by nature; and being an interference with nature is therefore unjust.

But this is the only mention of this idea in Aristotle's works and appears in the context of Aristotle arguing for the existence of natural slavery, and the "others" who hold this idea are not identified. It at least shows opposition to slavery existed in some coherent form.