The expulsion of the Pisistratids from power brought the confrontations between aristocrats back to the foreground, their expressers being on the one hand Isagoras, and on the other those who were in favour of broader power for the demos, headed by Cleisthenes the Alcmaeonid. In 508-7 B.C., with Isagoras archon at Athens, Cleisthenes managed to get voted through the replacement of the four tribes of Attica by ten new tribes and the increase of the members of the Council from four to five hundred. Each tribe was made up of three trittyes: one from the Asty, one from the Mesogia, and one from the Paralia. Each trittys in turn contained one or more demes. The restructuring of the area of the city had as its aim to upset the old division of citizens into groups according to place of residence, and also to undermine the families' political influence. In future the citizens would take their names from their deme of origin, thus abolishing the distinction between old Athenians and new citizens. From each tribe fifty councillors annually were chosen by lot, and these were 'in presidency' for one tenth of the year, going by the new civic calendar. The Assembly prepared the preliminary agendas of laws which were to be submitted to the Assembly of the demos. The Assembly - perhaps for the first time - was appointed to convene at regular intervals. In order to preclude further attempts, moreover, to impose tyranny, it is argued by some scholars that he introduced the measure of ostracism.

Invoking the Curse of Cylon, Isagoras asked Cleomenes king of Sparta to intervene. Cleisthenes was now forced to leave Athens, and at the same time seven hundred Athenian families were exiled as being 'under the curse'. But the Council resisted the abrogation of the reforms, and the demos besieged the Spartan force on the Acropolis. Two days later, by agreement, the Spartans left Athens and the supporters of Isagoras were lynched. Cleisthenes returned, though without keeping his power, but remained simply the "protector of the Demos". Meanwhile, Cleomenes had gathered a powerful force and had advanced to Eleusis, while the people of Boeotia and Chalcis launched a simultaneous attack. But disagreement between the two Spartan kings led to the disintegration of the army. The Athenians now landed, bent on revenge, in Euboea, having defeated the Boeotians, and attacked the people of Chalcis. After their victory they shared out the lands of the nobles of Chalcis to four thousand Athenians. These were the original cleruchs, for despite their redeployment they kept their civic rights. According to Herodotus, the Athenians' dominance is directly linked to Cleisthenes' introduction of isegoria, freedom of speech in the Assembly of the Demos. Every hoplite now fought with greater valour, because he felt that collective victory served his own personal interest.