The 13-year-old boy was killed when an SSF agent hit him on the head with a truncheon, according to eye-witnesses.
Clashes between anti-government protesters and security forces in Mahabad erupted on Wednesday evening, but reached their peak on Thursday night.
Thousands of residents took to the streets to chant slogans against the ruling authorities, witnesses reported. SSF agents attacked the crowd but protesters, who were mainly youths, took part in hit-and-run clashes which spread to several districts of the city.
Government buildings and patrol cars were attacked after news spread of the death of the teenage boy. A number of SSF agents were beaten up by demonstrators.
Dozens of protesters were arrested during the unrest or in the house-to-house searches that security forces undertook in several neighbourhoods after midnight. No information is available on their fate.
Protests erupted again in some districts of Mahabad for the third consecutive night on Friday, with young people chanting slogans against the regime and sealing off several streets with burning car tires.
Protests were also reported in the nearby city of Marivan, near the Iran-Iraq border.
Clashes in Marivan occurred yesterday, as young people in the citys Edareh Bargh Street challenged a SSF officer for accosting a girl. Local people took part in the ensuing fight and chanted anti-regime slogans and smashed the windscreens of several patrol cars.
A number of people there were reportedly arrested.
