Two instances: both episodes took place c.412 or 411 BCE, when the Peloponnesian War spilled over into the Eastern Aegean and the cities there began to reject Athenian authority. The first took place on Samos, the second (which took place chronologically earlier) on Chios.
At this time on Samos, the demos* rose up against the ruling class with the support of Athenians who were there with three ships. The Samian demos executed two hundred from elite and condemned four hundred more to exile, distributing amongst themselves their land and homes. After this, the Athenians decreed them autonomous. Henceforth they governed the city, excluding the (previously) prominent men from governance and forbidding intermarriage between them and members of the demos.
Ἐγένετο δὲ κατὰ τὸν χρόνον τοῦτον καὶ ἡ ἐν Σάμῳ ἐπανάστασις ὑπὸ τοῦ δήμου τοῖς δυνατοῖς μετὰ Ἀθηναίων, οἳ ἔτυχον ἐν τρισὶ ναυσὶ παρόντες. καὶ ὁ δῆμος ὁ Σαμίων ἐς διακοσίους μέν τινας τοὺς πάντας τῶν δυνατωτάτων ἀπέκτεινε, τετρακοσίους δὲ φυγῇ ζημιώσαντες καὶ αὐτοὶ τὴν γῆν αὐτῶν καὶ οἰκίας νειμάμενοι, Ἀθηναίων τε σφίσιν αὐτονομίαν μετὰ ταῦτα ὡς βεβαίοις ἤδη ψηφισαμένων, τὰ λοιπὰ διῴκουν τὴν πόλιν, καὶ τοῖς γεωμόροις μετεδίδοσαν οὔτε ἄλλου οὐδ᾽ ἐς ἐκείνους οὐδενὶ ἔτι τοῦ δήμου ἐξῆν.
*Note: demos is a somewhat loaded term since it can mean the citizen body. Here there is clear differentiation between the super wealthy and the majority. The redistribution of land indicates that Samos was experiencing a consolidation of wealth in the hands of a few, but the extent of this is unknown.
Straightway after the naval battle (Aegospotami) the rest of Hellas deserted the Athenians, save the Samians, who had gained mastery over the polis by carrying out a slaughter of the prominent men there.
εὐθὺς δὲ καὶ ἡ ἄλλη Ἑλλὰς ἀφειστήκει Ἀθηναίων μετὰ τὴν ναυμαχίαν πλὴν Σαμίων: οὗτοι δὲ σφαγὰς τῶν γνωρίων ποιήσαντες κατεῖχον τὴν πόλιν.
Xenophon, Hell. 2.2.6
The reason that they sent these ships was that the majority of the Chians were ignorant of the arrangements. The oligarchs and those in the know were not yet willing to bring war to the majority before they secured their position and because of the delay no longer expected the Peloponnesians to arrive.
αἴτιον δ᾽ ἐγένετο τῆς ἀποστολῆς τῶν νεῶν οἱ μὲν πολλοὶ τῶν Χίων οὐκ εῖδότες τὰ πρασσόμενα, οἱ δὲ ὀλίγοι καὶ ξυνειδότες τό τε πλῆθος οὐ βουλόμενοί πω πολέμιον ἔχειν, πρίν τι καὶ ἰσχυρὸν λάβωσι, καὶ τοὺς Πελοποννησίους οὐκέτι προσδεχόμενοι ἥξειν, ὄτι διέτριβον.
The episode on Chios is not class warfare in the same sense as the episode on Samos was, but very clearly indicates a conflict between the few and the many. Here, the conspirators hoped to lead Chios into revolt against Athens, but were waiting on promised aid from Sparta before making their appeal and therefore sacrificed a squadron of seven ships as a way to avert Athenian suspicion about their motives just a little longer. Thucydides’ phrasing here is interesting. The conspiracy will bring war to Chios against the will of the majority, but it is a close step from that to the conspirators bringing war against to the majority.