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 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.
Rami Abdul Rahman, head of the Syrian Observatory for Human Rights, urged the country's authorities to investigate the torture to death of the 13-year-old boy and other similar cases.
Seven bodies with torture marks, some with broken necks, were taken to Daraa hospital, he told AFP by telephone in Nicosia, adding that hundreds of cases of torture were being documented.
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.
Al Jazeera reported that after receiving his body, Khatib’s family was visited by Syrian secret police, who arrested the boy’s father. The boy’s mother said officers ordered her husband to say the boy was killed by armed Salafists, or ultra-conservative Muslims, whom Assad has claimed as being behind the unrest. She said the secret police had warned her not to speak to the press, threatening, “You know what would happen if we heard you had spoken to the media.”
Reuters, Ya Libnanvia yalibnan.com