BEIRUT—The images grow no less shocking with time—a gaping wound on a tiny skull, the hair matted with blood; a gunshot that pierced the skin of a small torso and went straight toward the kidney; and finally, the broken neck and severed penis of a 13-year-old boy, his mangled body contorted on a plastic sheet.

The images of children killed in a government crackdown on protests are circulating widely among Syrians on YouTube and Al-Jazeera, Facebook and opposition web sites. And they are stoking even more fury against a regime the opposition says has lost all legitimacy.

Syria’s government tried to blunt the anger with promises Wednesday to investigate the killing of 13-year-old Hamza al-Khatib , who tortured and mutilated body turned him into a symbol of the Syrian uprising.

But protesters deride that and other government concessions, including an amnesty that freed political prisoners and a committee to prepare for national dialog, as nothing more than a ploy to buy time for President Basher Assad. They say at least 25 children are among more than 1,000 dead, with government crackdowns that increase the toll almost daily.

The deaths of two girls—a 12-year-old killed on Saturday when her school bus came under fire, and an 11-year-old shot to death on Tuesday, while her town was being shelled—appeared certain to inflame tensions. Already, a Syrian opposition page refers to the older girl, Hajar Tayseer al-Khatib, as “the flower of Syria’s martyrs.”

Military operations in southern and central Syria killed at least 33 people on Tuesday and Wednesday, even as the government released hundreds of political prisoners. The government claims the revolt is the work of Islamic extremists and armed gangs.

Rami Abdul-Rahman, director of the Syrian Observatory for Human Rights, told The Associated Press that more than 500 prisoners were freed, including some who took part in the latest demonstrations marking the most serious challenge to the Assad family’s 40-year rule. Activists say 10,000 have been rounded up since the protests began mid-March.

Syrian state television on Tuesday said the amnesty covered “all members of political movements,” including the outlawed Muslim Brotherhood, which led an armed uprising against Assad’s father in 1982. Membership in the party is punishable by death.

Both the US and France said the amnesty would not be enough.

Leading Facebook page The Syrian Revolution 2011 addressed the regime directly in a posting: “Why do you hate our children. They are the symbol of our revolution. They are our freedom and the future of our country.”

Many of the dead children were from the southern province of Daraa, where the uprising was touched off by the arrest of 15 teenagers who scrawled anti-regime graffiti on the walls of the provincial capital. At the time, those teens became the symbol of the new revolt inspired by the toppling of regimes in Egypt and Tunisia.

Hamza, the 13-year-old boy, has become a new emblem of the uprising and thousands of people now carry his smiling photo during protests or post it as their Facebook profile. A Facebook page set in his memory has more than 66,000 fans.

Al-Jazeera did not air the whole video, but a copy posted by opposition on YouTube showed that the boy’s penis was severed and his neck broken. The body, lying on a plastic sheet, appeared pink and the eyes were mottled with bruises and black marks.

Opposition groups blamed security forces for the boy’s death.