The Romans wanted blood, and there was plenty to be had at the annual game fixtures and at special events celebrating triumphs, festivals, and political campaigns. The biggest and best of the annual tournaments was the state sponsored Ludi Romani or "Roman Games," which were the main Roman pastime every September.
According to legend, the Ludi Romani were established by Tarquinius Priscus, Rome's fifth king (616-578 BC), to celebrate one of his local military victories. Tarquin also was said to have laid out the Circus Maximus and allowed the upper classes to build private grandstands. In Tarquin's time, the Ludi were only a one-day event featuring boxers and horse races, but they had grown to ten days late in the Republic and to fifteen days by the time of Julius Caesar. A sixteenth day was added in his honor after Caesar was killed, and from then on the games were held from September 4 through 19. Somewhere along the way, Rome's beloved blood sports became part of the celebration, but the focus was on the chariot racing. There was a short time-out each September 13 for the Epulum Iovis, a feast honoring Jupiter. The games also started with a religious observance, a procession from the Capitoline Temple of Jupiter to a sacrificial altar in the Circus Maximus.
The Ludi Romani, in their fully developed form, featured ludi circences, which were chariot races, boxing, and gladiatorial contests, in all the sports venues. The Circus Maximus, where the immensely popular chariot races were held, could seat more that 250,000 spectators. The Colosseum held more than 60,0000, and smaller venues another 50-60,000. Simultaneously, ludi scaenici, theatrical competitions featuring Greek and Roman classics and new plays, were held in all the theaters, but they necessarily played to smaller audiences.
So how bad -- how barbaric -- were the games? Well, the ludi scaenici were pretty tame. There was little of the gore and grief of Greek tragedy. Romans liked comedies and especially the farces of Plautus. Many of Plautus' plays are still available and several of them formed the basis of the 20th century Broadway musical and the movie A Funny Thing Happened on the Way to the Forum.
The ludi circenses, on the other hand, were probably bloodier than anything we can imagine. The chariot races were much more dangerous than what you've seen in Ben Hur and other Hollywood classics. The chariots were flimsy wicker affairs offering no real protection to drivers. They didn't play fair, so crashes and fatalities among the drivers (followed sometimes by riots by their fans) were common. Horses were yoked or hitched with traces rather than collared, so it wasn't too unusual for them to falter or suffocate and die during races. Winning horses were sometimes slaughtered on the spot in quasi-religious ceremonies. One annual ritual of the Equirria games, which were held in the Circus Flaminius on the Campus Martius each February 27, called for the inside horse of the winning team of the feature race to be killed and decapitated. Then the audience would split up by neighborhoods and fight over the head on the floor of the Circus.
The gladiatorial contests over in the Colosseum, whether against animals or other men were, of course, much worse. Accounts of some thousands of animals and men killed over a period of several days during big late-empire game fixtures and of amphitheater floors running with blood are not exaggerated. Almost all the human and animal victims were killed with bladed weapons. Criminals, including recalcitrant Christians, were often simply executed by sword stroke during the noon break when most of the audience was out to lunch. Even much criticized movie and television blood and violence is minor and understated compared to the reality of what Roman parents almost daily took their kids to watch. No one kept a running total of all the human dead -- it apparently didn't matter to anyone -- so estimates vary wildly. Certainly at least hundreds of thousands were slaughtered just in Rome. But the actual numbers, when discussing such widespread public barbarity, don't really matter: Romans of all classes were enamoured of the bloody spectacles, and there always seemed to be enough victims, even if some had to be imported.
Those who objected were few, and they had little influence to stop the games. Some emperors, Hadrian, for example, reportedly hated the bloodshed but did not know how to shut it down without risking riot. It wasn't until long after Constantine that the blood games were able to be suppressed in Rome, and then only because the city had gotten well along in its dramatic population decline caused by his decision to move the capital to Constantinople.
Ludi Romani from the Atrium: http://web.idirect.com/~atrium/thisday/features/ludiromani.html
Ludi from Nova Roma: http://novaroma.org/ludi/index.html
Livy on Tarquin and the Ludi Romani: http://www.perseus.tufts.edu/cgi-bin/ptext?doc=Perseus%3Aabo%3Aphi%2C0914%2C001&query=1%3A35
Gladiatorial games from CUNY: http://depthome.brooklyn.cuny.edu/classics/gladiatr/index.htm
Gladiatorial games from Vroma: http://www.vroma.org/%7Ebmcmanus/arena.html
Chariot Racing from Vroma: http://www.vroma.org/%7Ebmcmanus/circus.html
Chariots and Racing: http://www.mmdtkw.org/VSweetChariot.html