Tiberius Gracchus' overruling of the tribunician veto was considered illegal, and his opponents were determined to prosecute him at the end of his one year term, since he was regarded as having violated the constitution and having used force against a tribune. To protect himself further, Tiberius Gracchus sought re-election to the tribunate in 133 BC, promising to shorten the term of military service, abolish the exclusive right of senators to act as jurors, and admit allies to Roman citizenship.
As the voting proceeded, violence broke out on both sides. Tiberius' cousin, Publius Cornelius Scipio Nasica, saying that Tiberius wished to make himself king, led the senators down towards Tiberius. In the resulting confrontation, Tiberius was beaten to death with the chairs of the senators and thrown into the Tiber.[21] Several hundred of his followers, who were waiting outside the senate, perished with him. Plutarch says, "Tiberius' death in the senate was short and quick. Although he was armed, it did not help him against the many senators of the day."
http://en.wikipedia.org/wiki/Tiberius_Gracchus