An injured man is treated in Deraa April 8, 2011 in this still image taken from amateur video, (Reuters).
Syrian security forces in at least two towns prevented medics from reaching wounded protesters when clashes erupted at anti-government demonstrations last week, Human Rights Watch (HRW) said.
The New York based rights group said the "inhumane" and "illegal" blocking of access to medical treatment occurred in the southern town of Daraa, the centre of a wave of protests against President Bashar al-Assad, and Harasta near Damascus.
"Barring people from needed medical care causes grave suffering and perhaps irreparable harm," said Sarah Leah Whitson, HRW's Middle East director.
"To deprive wounded people of critical and perhaps life-saving medical treatment is both inhumane and illegal."
She urged Syrian authorities to allow injured protesters unimpeded access to medical treatment and to stop using unjustified lethal force against anti-government protesters.
The rights group said it had interviewed doctors as well as injured protesters and their relatives in Daraa and Harasta and the town of Douma.
It said a total of 28 people were killed in the three towns on Friday when security forces fired on protesters.
"Syria’s leaders talk about political reform, but they meet their people’s legitimate demands for reform with bullets," said Whitson.
Witnesses in Daraa said several thousand protesters marching after the weekly Muslim main prayers on Friday carrying olive branches.
When they approached a roadblock manned by "several thousand" security force members they were ordered to halt, the witnesses said.
Security forces then fired teargas and live rounds, while snipers on roofs opened fire.
HRW quoted witnesses as saying the security forces did not allow ambulances to approach the road to pick up the wounded, and kept shooting when other protesters tried to carry the wounded away.
It said the April 8 shootings brought the total number of protesters killed in Daraa and surrounding villages since March 18 to at least 130, citing lists compiled by Syrian human rights groups and its own investigations.
On the same day in Harasta, HRW said, about 2,000 protesters left the main mosque after Friday prayers, also carrying olive branches.
They were met by a large group of security forces blocking the road. The two sides threw rocks at each other, the protesters said, adding that men in plain clothes emerged suddenly from a side street and opened fire with Kalashnikovs, with no warning.
Two doctors told Human Rights Watch that they each treated four wounded protesters in Harasta. They said it was impossible to bring the injured into the hospital because it was surrounded by the security forces.
"I was in the hospital in the afternoon, when I started getting calls from people asking for help," one doctor said.
"I knew people could not bring the wounded in -- the hospital was surrounded by the security personnel. We also couldn’t send an ambulance, fearing the security forces would open fire, as happened in other places."
The doctor said he rushed to private homes where the rescuers brought the wounded.
"I could not take any major supplies or tools; only the most basic things," the doctor said.
"The injuries were serious and we had nothing to work with -- in one case, we had to probe a wound with a metal spoon to see how deep the bullet went."
Another doctor said that five protesters with bullet wounds came to the house he was in, and that he knew of at least six other houses where doctors treated wounded people.