The ancient Jewish court system was called the Sanhedrin. The Great Sanhedrin was the supreme religious body in the Land of Israel during the time of the Holy Temple. There were also smaller religious Sanhedrins in every town in the Land of Israel, as well as a civil political-democratic Sanhedrin. These Sanhedrins existed until the abolishment of the rabbinic patriarchate in about 425 C.E.
The earliest record of a Sanhedrin is by Josephus who wrote of a political Sanhedrin convened by the Romans in 57 B.C.E. Hellenistic sources generally depict the Sanhedrin as a political and judicial council headed by the country’s ruler.
Tannaitic sources describe the Great Sanhedrin as a religious assembly of 71 sages who met in the Chamber of Hewn Stones in the Temple in Jerusalem. The Great Sanhedrin met daily during the daytime, and did not meet on the Sabbath, festivals or festival eves. It was the final authority on Jewish law and any scholar who went against its decisions was put to death as a zaken mamre (rebellious elder). The Sanhedrin was led by a president called the nasi (lit. “prince”) and a vice president called the av bet din (lit. "father of the court"). The other 69 sages sat in a semicircle facing the leaders. It is unclear whether the leaders included the high priest.
The Sanhedrin judged accused lawbreakers, but could not initiate arrests. It required a minimum of two witnesses to convict a suspect. There were no attorneys. Instead, the accusing witness stated the offense in the presence of the accused and the accused could call witnesses on his own behalf. The court questioned the accused, the accusers and the defense witnesses.
The Great Sanhedrin dealt with religious and ritualistic Temple matters, criminal matters appertaining to the secular court, proceedings in connection with the discovery of a corpse, trials of adulterous wives, tithes, preparation of Torah Scrolls for the king and the Temple, drawing up the calendar and the solving of difficulties relating to ritual law.
In about 30 C.E., the Great Sanhedrin lost its authority to inflict capital punishment. After the Temple was destroyed, so was the Great Sanhedrin. A Sanhedrin in Yavneh took over many of its functions, under the authority of Rabban Gamliel. The rabbis in the Sanhedrin served as judges and attracted students who came to learn their oral traditions and scriptural interpretations. From Yavneh, the Sanhedrin moved to different cities in the Galilee, eventually ending up in Tiberias.
Local Sanhedrins consisted of different numbers of sages, depending on the nature of the offenses it dealt with. For example, only a Sanhedrin of 71 could judge a whole tribe, a false prophet or the high priest. There were Sanhedrins of 23 for capital cases and of three scholars to deal with civil or lesser criminal cases.
The modern “Sanhedrin Initiative” is a rabbinic effort to restore the ancient Jewish Supreme Court by renewing classical semichah (Biblical ordination), which is based on Maimonides’ ruling that the consensus of Israel’s sages could reinstate it. It explains the historical loss of semichah about 1,600 years ago, the failed Safed attempt of 1538 led by Rabbi Yaakov Beirav and Rabbi Yosef Karo, and later attempts in the 19th and 20th centuries. The current process, launched in 2004, involved widespread outreach to hundreds of leading Israeli rabbis and culminated in the ordination of Rabbi Moshe Halberstam as the first samuch, followed by others, with noted support or non-opposition from central Torah authorities. While some rabbinic disagreement remains over the halachic validity of the method, supporters argue it is a unifying, non-political effort to restore Jewish legal leadership and moral authority in Israel.

Sources: Blackman, Philip. Introduction to Tractate Sanhedrin of the Mishnah. New York: The Judaica Press, 1963; Dimont, Max. Jews, Jews, God and History. New York: The New American Library, 1962; Encyclopedia Judaica “Sanhedrin”. Jerusalem: Keter Publishing House, 1971; Kung, Hans. Judaism. New York: Crossroad, 1992; Seltzer, Robert M. Jewish People, Jewish Thought. New York: Macmillian Publishing Co, 1980.
“The Sanhedrin Initiative”, thesanhedrin.org.

Ancient Jewish History: The Sanhedrin