BAGHDAD — Iraq’s supreme religious authority condemned Friday the gruesome lynching of a teenage boy by anti-government demonstrators in Baghdad, calling it a “heinous crime” that could torpedo their weeks-long protest movement if the perpetrators are allowed to go unpunished.

Grand Ayatollah Ali Sistani, Iraq's top Shiite Muslim cleric, said in his weekly sermon that such violence threatens the security and stability of the country, and he called on Iraqi authorities to seek out and punish those responsible for the crime. The ayatollah holds powerful sway over Iraq's majority Shiite population.

Demonstrators and local youth have occupied a swath of central Baghdad as part of their efforts to oust the government amid calls for sweeping reform. In recent weeks, protesters and activists have been killed, kidnapped and fired on by masked gunmen, contributing to a climate of paranoia and fear.

The killing took place early Thursday after 15-year-old Maitham Ali Ismail, a local resident, reportedly fired a pistol in the air to disperse protesters camped out near his home. In an interview with a local television station, his grandmother said the crowd had been making noise and burning tires, causing damage to the house.

A mob then reportedly descended on his neighborhood, dragged Ismail out and stabbed and mutilated his body before hanging him by his ankles from a traffic light in the square. Images circulating on social media showed crowds of young men recording the lynching on their cellphones.

Other videos, which could not be independently verified, purportedly showed riot police assisting demonstrators in their hunt for Ismail. In the footage posted online, men wearing police uniforms fired guns and smoke grenades into homes in the area near Wathba Square, where Ismail was hanged, while youths cheered them on.

Police said Friday they arrested one suspect in connection with Ismail’s murder. A video of the detainee’s alleged interrogation also surfaced online.

Iraq’s Supreme Human Rights Commission said in a statement that security forces were observed watching the lynching from the sidelines and that they had “failed to perform their duties.” The commission denied reports that Ismail had opened fire on the crowd, killing several protesters. It said its investigation found that he only fired into the air, and the Baghdad central morgue said it received just one body — that of the teenager.

A statement published online and read over loudspeakers at the main protest site in Tahrir Square condemned the killing and those who participated in it.

“It was tragic what we saw: people gathering to watch these heinous scenes yesterday,” Sistani said his televised sermon, which was read by one of his aides.

“The judiciary should be the institution responsible for dealing with all crimes. Dragging, hanging and mutilating a body — these are all crimes,” Sistani said.

Nearly 500 people have been killed since anti-government protests began in Baghdad and other cities in October. Demonstrators are calling for a radical overhaul of the political system, which they say nurtures corruption and favors an out-of-touch elite.

Security forces and allied militias have fired on and arrested protesters. Some activists have been targeted for assassination in Baghdad and in Karbala, roughly 75 miles to the south.

In a report this week, the United Nations said it received “credible allegations of deliberate killings, abduction and arbitrary detention carried out by unknown armed men.”

Human rights violations and abuses continued during the period covered by the report, the United Nations said, including unlawful and excessive use of force, as well as ill treatment of arrested demonstrators.