But they risk arrest by doing so. Two women were taken in broad daylight from a cafe last month, as was the son of a doctor this month, for helping stockpile and deliver medicine, activists say. Another man was arrested after collecting Easter chocolate to send to Christian children in Homs and was held for several weeks. The authorities have even detained a psychiatrist who was training volunteers to help children who have been traumatized by an uprising that has churned for more than a year.

“They want to get rid of the idea that the people can help each other,” said a social scientist who is a participant in the network and, like others interviewed, did not want to be identified. “They don’t want there to be solidarity among the Syrian people.”

Those involved say they were moved to act by the realization that the government had no intention of responding to the humanitarian crisis caused by its own siege of Syrian cities, which has resulted in the internal displacement of hundreds of thousands of people. Those with the means have filled Damascus hotels, which have offered drastically reduced rates to Syrians fleeing the violence, and those without have been welcomed in the homes of less affluent Syrians on the outskirts of Damascus.

Providing relief is “not disallowed in law, but we understood from the security that it was illegal,” said one participant who noted that the most dangerous items to smuggle were medicines. “We began to fear for our people so we stopped medicine, stayed to food.”

Every stop on what has become a sort of underground railway is shrouded in secrecy: requesting supplies, confirming the needs, delivering goods, fund-raising and collecting donations are carried out by separate cells. To safeguard the network, participants know the identities of only the immediate people they deal with. Members estimate that there are hundreds of people working in the network in greater Damascus and that well into the thousands within Syria are contributing money and supplies.

But people who want to help say they have no choice but secrecy, citing the results of an attempt in March to openly organize an aid caravan to Homs.

The organizers created a nonpartisan Facebook page called “Carrying Homs in Our Hearts,” with a logo of a hand holding an olive branch. The page identified organizers’ real names and phone numbers as people who would receive donations to take to Homs.