The body of Hamza al-Khatib was returned to his family on Wednesday, following his disappearance after a demonstration on April 29, activists said on their Facebook site, Syrian Revolution 2011.
"We will go out from every home, from every district to express our anger" over the killing, they wrote on the page which carries a picture of the boy.
"A month had passed by with his family not knowing where he is, or if or when will he be released. He was released to his family as a dead body. Upon examining his body, the signs of torture are very clear," they said.
"There were a few bullets in his body used as a way of torture rather than to kill him with. Clear signs of severe physical abuse appeared on the body such as marks done with hands, sticks, and shoes. Hamza’s penis was also cut off."
Other activists said Hamza al-Khatib decided after police had killed his cousin to take part in the anti-regime protests sweeping the country since mid-March, with their epicentre in the Daraa region of southern Syria.
His father, Ali al-Khatib, has also been arrested, they said.
On Friday, at least 12 people were shot dead as security forces dispersed protests across Syria, activists said, updating an earlier toll of eight dead.
Four protesters were killed in Daraa, another four in a Damascus suburb, three in Homs, central Syria, and one in Latakia on the coast, said the London-based National Organisation for the Defence of Human Rights.
Since the revolt in Syria erupted, Friday protests following weekly Muslim prayers are widely seen as a barometer of whether activists are able to maintain momentum despite a heavy handed repression.
More than 1,000 people have been killed and 10,000 others arrested since the revolt began, according to rights groups. Syrian authorities say 143 soldiers, security forces and police have been killed.
Foreign journalists are barred from travelling inside Syria, making it difficult to report on the unrest and verify witness accounts.
The government insists the unrest is the work of "armed terrorist gangs" backed by Islamists and foreign agitators.
It initially responded to the revolt by offering some concessions, including lifting the state of emergency in place for nearly five decades, but coupled this with a fierce crackdown.
The opposition has dismissed calls for dialogue, saying that could only take place once the violence ends, political prisoners are freed and reforms adopted.