The Syrian Observatory for Human Rights, a Britain-based group opposed to the Syrian regime, says Mohammad Qataa was shot in the face and neck a day after being seized.

It has released a photo showing Qataa's face with his mouth and jaw bloodied, as well as a bullet wound in his neck.

"The observatory cannot ignore these crimes, which only serve the enemies of the revolution and the enemies of humanity," the group's leader Rami Abdulrahman said.

The observatory based its report on witness accounts of the killing.

It says Qataa was a street vendor selling coffee in the working-class Shaar neighbourhood.

He has reportedly been arguing with someone when he was overheard saying: "Even if the Prophet Mohammad comes down (from heaven), I will not become a believer."