Shocking video footage has emerged from the Syrian city of Homs in which a young man filming gunfire in the streets appears to be shot dead in cold blood by the sniper he zooms in on.

A clip circulating on YouTube begins with a male voice describing "someone shooting at citizens in Karm al-Sham on 1 July without any reason and no demonstrations."

The cameraman is filming from an upper floor against a background of slogans being chanted. Jerky images of the street and balconies are followed by a blurred glimpse of a man in olive green, standing in the shadows, carefully moving forward and raising and firing a weapon – followed by a single shot, moaning, and distraught voices pleading for help.

The cameraman's identity is not known. Foreign journalists and human rights groups are largely banned from Syria and it has not been possible to authenticate the video.

The caption describes the gunman as a member of the Shabiha, a militia used by the Assad regime. Last Friday's demonstrations were described as the biggest yet in the three and a half month uprising. Human rights groups say the death toll in Homs, Syria's third city, is continuing to rise as security forces and gangs loyal to the Assad regime seek to crush protesters who come out in growing numbers in separate neighbourhoods on a daily basis. Tanks remain positioned in the city.

Separate films posted online on Saturday and Sunday appear to show the killing of a young man named as Diyya al-Najjar when security forces opened fire on protesters in the al-Qarabis neighbourhood of Homs. Crowds are seen running and scattering as gunfire rings out. One young man is shot in the middle of the street, as two men point weapons from the cover of parked cars. A witness told Human Rights Watch: "I saw Diyya al-Najjar shot by a sniper in his head right in front of me. The sniper was in a Land Cruiser car four or five metres away from protesters."

Najjar was described by some Syrian sources as an activist and blogger. His body was taken to al-Barr hospital in Homs, where a doctor confirmed to Human Rights Watch that he died from a bullet to the head. According to the doctor, 10 protesters wounded by bullets had arrived at his hospital by 6pm on Friday.

Video showed the bodies of two men identified as al-Najjar and Bassam Saqeene. Al-Najjar's face was surrounded with flowers, his body wrapped in the Syrian flag. Saqeene was killed in Homs on Thursday, according to Syrian opposition sources.

• Nidaa Hassan is a pseudonym for a journalist working in Damascus