In his March 1850 address of the Central Committee to the Communist League, Marx states that:
"To be able forcefully and threateningly to oppose this party, whose betrayal of the workers will begin with the very first hour of victory, the workers must be armed and organized. The whole proletariat must be armed at once with muskets, rifles, cannon and ammunition, and the revival of the old-style citizens’ militia, directed against the workers, must be opposed. ... any attempt to disarm the workers must be frustrated, by force if necessary. The destruction of the bourgeois democrats’ influence over the workers, and the enforcement of conditions which will compromise the rule of bourgeois democracy, which is for the moment inevitable, and make it as difficult as possible – these are the main points which the proletariat and therefore the League must keep in mind during and after the approaching uprising."
What sort of organization was Marx talking about, and what makes them "old-style"? Was Marx trying to contrast them to more contemporary organizations that he did not consider objectionable, emphasizing that militias in general were defunct, or just invoking the "bad old days" for rhetorical flair?
Prior to the development of modern policing, traditionally dated to Peel's reforms that started in 1829, the troops most likely to be called out to suppress riots and insurrections - pursuant to powers granted to local magistrates under the Riot Acts of the period, which were the legal power of choice for when the lower classes got unruly - would be local yeomanry units, volunteer local-defence cavalry formed from among those who could afford to supply their own horses. As an institution they evolved into what we now call the Territorial Army, and in theory their role was to muster to be ready for when the French came across the channel. In practise, the only action they ever saw was training, occasional parades, and turning out to put down riots. At first as the only option, and later, after 1829, as an escalation from what the regular constabulary could do.
The first deaths at the Peterloo Massacre - in 1850 well within recent political memory - were inflicted by a Yeomanry unit who were first on the scene, which I pick out because it's one of the most famous incidents and the subject of a current feature film. (http://www.peterloomassacre.org/history.html for a summary of events by campaigners for a permanent memorial.)
Within closer political memory as at 1850 would be the Chartist risings of the 1840s, which came to military action several times, and again the first troops available to local magistrates under the Riot Acts were the Yeomanry units, or, later, the mass drafts of special constables.
Those events would have been familiar to an audience in London in 1850, at least as to the natives present. Someone who knows the history of public-order law on the Continent could perhaps speak to whether Marx himself - who'd only just recently moved to England - would have been saying something familiar to the expatriates present.