Analysts caution that the strife in Homs is still specific to the city itself, and many in the opposition reject violence because they fear it will serve as a pretext for the government’s brutal crackdown.

But in the targeted killings, the rival security checkpoints and the hardening of sectarian sentiments, the city offers a dark vision that could foretell the future of Syria’s uprising as both the government and the opposition ready themselves for a protracted struggle over the endurance of a four-decade dictatorship…

An armed opposition is battling security forces in the most restive neighborhoods. Insurgents have tried to protect the same peaceful protesters the government has relentlessly sought to arrest. Tension has grown so dire that members of one sect are reluctant to travel to neighborhoods populated by other sects. Men in some parts of the city openly carry weapons.

Perhaps the most dramatic facet of the struggle is a series of assassinations this past week that have left nearly a dozen professors, doctors and informers dead in a paroxysm of violence that echoes the sectarian vendettas still besetting Iraq. Unlike the uprising’s early days, when the government exercised a near monopoly on violence, fear is beginning to spread in the other direction, as insurgents kill government supporters and informers, residents say.