Year 121 BC was a year of the pre-Julian Roman calendar. At the time it was known as the Year of the Consulship of Opimius and Allobrogicus (or, less frequently, year 633 Ab urbe condita). The denomination 121 BC for this year has been used since the early medieval period, when the Anno Dominicalendar era became the prevalent method in Europe for naming years.
The Roman Senate passes the motion senatus consultum ultimum, which the consulLucius Opimius interprets as giving him unlimited power to preserve the Republic. He gathers an armed force of Senators and their supporters to confront Gaius Gracchus. A pitched battle is fought inside Rome, resulting in the death of Gracchus and many of his followers.
A tribunal is established in Rome that executes 3,000 followers of Gracchus.