Old Testament elders could represent a city or town (Deuteronomy 19:12, 1Samuel 16:4), the people of a particular tribe (Numbers 22:7, Deuteronomy 31:28, 2Samuel 19:11), a family (2Samuel 12:17) or a country (Joshua 9:11). They were eligible to sit on the Sanhedrin, or properly the Great Sanhedrin, which met in Jerusalem and was the supreme council of the Jews.
The word translated as elder in the New Testament is the Greek word presbuteros (Strong’s Concordance #G4245), which generally is used to refer to older men respected by others as leaders and role models. The elders of the people were some of the most vehement adversaries against Jesus and his teachings. They used their power to help insure a plan to have Jesus arrested, tried, convicted and sentenced to death was successful. Their active role in the New Testament during the last few days of Jesus' life includes the following.
It was known, in advance, that the elders would play a big part in causing the suffering and death of Jesus (Matthew 16:21, Mark 8:31, Luke 9:22).
The elders, along with the chief priests and scribes, challenged Jesus' authority just days before the crucifixion (Matthew 21:23, Luke 20:1 - 2). They were among a group of leaders assembled at the High Priest's palace, just before the New Testament Passover, for the purpose of devising a way to have Jesus killed (Matthew 26:3 - 4). They, along with the chief priests, bribed Judas with thirty pieces of silver in order to betray Jesus. A sorrowful Judas, after Jesus' arrest, came back to those who bribed him and wanted them to take back their "blood money." They refused, but used the money to buy a potter's field - which fulfilled Bible prophecy (Matthew 26:14 - 16, 27:3 - 10; Acts 1:15 - 19).
These religious leaders also gave their authority, along with other Jewish leaders, to have Judas Iscariot escorted by armed men to the Garden of Gethsemane where Jesus could be arrested (John 18:1 - 9, Matthew 26:47). In the New Testament, they assembled with others, after the arrest of Jesus, to question him (Matthew 26:57). They sought false witnesses to testify against Jesus (v. 59 - 61). Ultimately, they voted to condemn him to death (verse 66).
It was they, along with the chief priests, who got a crowd of people to request a known murderer and robber named Barabbas be set free by the Romans instead of Jesus (Matthew 27:15 - 23). The elders were among those who mocked Christ as he was dying on the cross (Matthew 27:41).
They and other leaders decided, after they were told of the resurrection of Jesus, to BRIBE the Roman soldiers to lie about why his body was no longer in the Garden tomb (Matthew 28:11 - 15). They also played a significant role against the early church and against the apostle Paul in particular.
They, along with the High Priest and other, were furious that Peter and John were preaching the gospel to the masses. Peter and John, after being questioned as to why they are preaching Jesus and by what power a lame man was healed, were released (Acts 3:1 - 10, 4:5 - 6, 23).
They, with the chief priests, conspired with more than forty other Jews to have the apostle Paul murdered (Acts 23:12 - 15).
They and the High Priest traveled from Jerusalem to Caesarea in order to, before the Romans, testify against the apostle Paul and what he taught (Acts 24:1).
The early church also had leaders generally referred to as elders (Acts 11:30, 13:3, 20, 21, 1Timothy 5, James 5, etc.). These New Testament leaders served various functions in the church including pastors (Ephesians 4:11), overseers (Acts 20:28) and leaders and rulers of the flock (Hebrews 13:7, 1Thessalonians 5:12).