The Roman Senate, contrary to popular belief, was not at all a democratic body. It was simply what evolved from the assembly of elders that resembled the Senate in the days before the Republic. Senators were basically the heads of Roman families, initially being composed only patricians, but later expanding to include plebeians. They were selected by their families, not by any sort of popular vote.

Another common myth is that the Senate actually had legislative power. They didn't even have a salary. The only official power they had was to pass decrees (senatus consulta). The rest of their influence came from their massive social power. Laws passed by the comitia (general assemblies that did make laws) could be annulled if the Senate disapproved them. This massive social significance also manifested itself in such things as the murder of the Tiberius Gracchus by a mob instigated by Senators that disapproved of his populist reforms.

Since the number of Senators were based on the number of major families in Rome, the number of Senators grew with the population. From 100 initially, to 900 under Julius Caesar, to a maximum of 6000, divided between the Roman and Constantinopolian Senates.

Also, Senators weren't merely just Senators. They could hold several magistracies (offices that were elected), 2-3 being the canonical number, as well as own businesses and other private ventures. This integration with business also made them somewhat corrupt. Okay, extremely corrupt. Political campaigning was constant and ruthless. Around election time, riots might have been expected. Bribery was the rule, not the exception.

In general, the Roman Senate only remotely resembles any modern ones. Most of the similarities come from the fact that it was an assembly of old fogies talking about how everyone else should live their lives.