So before anybody tries to claim Clodius, whose violence really is not especially outstanding when the actual facts are compared with what is known of earlier incidents of political violence, in this particular category, I'm going to head that off by mentioning this

Besides that I'm afraid the answer is rather underwhelming. Political violence, which people often point to to legitimize characterizations of Republican "gang violence," should really not be considered at all, as it constitutes a particular form of organized violence, namely the use of force or the threat of force (often just as effective) in order to override and take possession of normal political procedure, most notably elections and legislative voting. There's something decidedly mafia-esque about the private retinues of supporters, dependents, clients, and hired muscle that surrounded every Roman of any social significance, but the analogy is quickly stretched too thin. Moreover, attempts to argue that the organized cores of political violence were anything but essentially private, very small groups are unconvincing and remain thoroughly unsupported by textual evidence. The case of Clodius is often used either as a model on which to base all arguments of political violence or as a revolutionary advance in the way that organized political violence was carried out, when in fact there's actually very little in the texts to support any such conclusion, and a more sober look at Clodius' violence reveals that the only thing remarkable about it is how much Cicero talked about it. Given the enormous amount of material from Cicero (several dozen speeches and about nine hundred letters) it should hardly be surprising that a political opponent of his would take up vastly more space than his predecessors and successors, and even then Cicero gives us almost no basis on which to judge Clodius' violence as unprecedented in its composition or methods. Therefore the view of violence in urban politics as developing, with Clodius, a truly "gangster" nature, whereby the city became ravaged by localized, urban organizations created purely for the exercise of political muscle should be discarded as having no evidence, and a not insubstantial amount of reasons to think that this was not the case.

Having cordoned off that particular form of political vis into a sort of category of its own, outside of what we modern observers think of when we talk about "organized crime" or "gangster violence" (which is not at all necessarily what a Roman meant by the crime of vis) we in fact find precious little evidence for purely "criminal" organizations in the city. There's no shortage of evidence that the city was a dangerous place, especially at night. Our texts, written entirely by literate, educated people, pretty much unanimously testify that urban dwellers did not go outside at night without some sort of entourage, even if only a slave or two for protection and to carry the light--but at the same time these same people didn't go anywhere in the daytime without similar retinues, although the very public aspect of such parades would be far more noticeable and socially useful in the day. Going through all the evidence is unfortunately not something that can be easily done on reddit (likewise for political violence, the evidence for which I have skipped for the sake of reddit's irritatingly restrictive space limits) but for all these references to hooligans and robbers in the city we don't get very much evidence for the presence of long-term, standing purely criminal organizations within the city. The presence of individual gangs romping and roaming does not tell us very much; one robber is nice, two is better, three is a real force to be reckoned with. The same principle of numbers applies today--bank robbers or even petty criminals frequently operate in groups, sometimes in quite large numbers, but rarely are their actions attributed to any larger organization. Surely many of these bodies, political and purely criminal, maintained themselves for some time and exerted pressure on local areas. Tacitus has a reference to a soldier who, prior to enlistment, was a "dux theatralium," a term which Tacitus seems to suggest was the leader of some kind of theater gang, although the nature of such organizations is highly unclear. Likewise Sallust has a similarly cryptic reference to "duces multitudinum," apparently the leaders of some bodies of the urban plebs who, along with the opifices (craftsmen) of the city, were sought out (apparently with no success) by the supporters of Cethegus and Lentulus. A simultaneously political (in a local sense and, in some way, in a larger sense) and criminal nature is implied here, but we lack the evidence to say very much more. We do not have evidence of shopkeepers paying "protection money" or anything like that--none of the usual signs of localized control by criminal organizations such as we recognize them now are present, and therefore we are left with a very unclear picture that it is not easy to fill out.

Evidence for large criminal organizations of a localized political nature (where the phrase "localized political nature" must mean the effective control of local regions by criminal organizations in lieu of regular state administration) outside the city is a little bit more readily available, but the picture becomes quickly complicated. Once again we have lots of evidence for localized bands of brigands and highwaymen, but while such groups often exert some amount of indirect control over the politics and administration of regions I think they can be effectively excluded from consideration. We're not trying to figure out whether groups of robbers terrified rural villagers into asking for help or hiding their belongings, we're trying to figure out evidence for the localized usurpation of state functions by strictly criminal organizations existing outside the state. When we look at extra-urban Italy and the provinces we should exclude petty crime, even on the scale of mass brigandage, and localized provincial rebellions, popular or otherwise. These aren't what we're looking for. When we define more precisely what it is we're looking for we're left with a picture no more conclusive than that of the city: there's no evidence. We don't have texts telling us that this or that local potentate was effectively in control of this or that location in Italy or the provinces, despite being unrelated to the functions of the state or of local administration, provincial or Italian. We can reasonably conjecture with a fairly high degree of certainty that local wealthy families and aristocracies usually dominated the local magistracies of various little Italian towns, but we cannot from there describe anything like a Pablo Escobar situation from our evidence. It's just not there.

An interesting outlier may present us with some evidence for a situation very broadly similar to the sort of localized usurpation of all administrative and economic power within a certain region by what are essentially large criminal organizations. Ignoring Holland's utterly ridiculous (and decidedly non-scholarly) belief that the Cilician pirates represented some sort of "counter-culture" (whatever he thinks he means by that) it's clear that the Cilicians were not a normal polity. In the wake of the Mithridatic Wars early in the first century the region of Cilicia became essentially devoid of any organized government that could be legitimately recognized as a "state," even by ancient standards. Piracy from Cilicia and Anatolia existed in a grand, expansive form, with large, organized fleets sailing as far as Ostia, raiding and pillaging the Italian and provincial coasts until Pompey's fabulously successful campaign against them. Plutarch describes the pirates in Cilicia as no longer being strictly pirates--they operated from fortified bases, with organized fleets and chains of command. The picture seems remarkably like one of a region in revolt, except that it's hard to categorize the Cilicians as being in revolt. Revolt against what? And what were they in revolt for exactly? Plutarch, however, also describes the pirates as funded by nobles and in particular Mithridates himself. The line of criminality now appears hesitatingly thin. On the one hand, the pirate fleets of the Cilician cities were manifestly criminal. Yet they were funded and organized by established state ties, both locally and apparently as agents of Pontic Anatolian hegemony. Should we understand them as fully independent or as instruments of a separate administrative control? There's more thinking to be done here (though again precious little evidence), but while there certainly appear to be some similarities the differences are also rather striking.