Asha Ibrahim Dhuhulow, 23, was buried up to her neck in front of hundreds of people Monday in a square in Kismayu and stones were hurled at her head. She was dragged out of the hole three times to see if she was dead.
Islamists said Dhuhulow wanted punishment under Sharia law.
"A woman in green veil and black mask was brought in a car as we waited to watch the merciless act of stoning," said Abdullahi Aden, who witnessed the execution. "We were told she submitted herself to be punished, yet we could see her screaming as she was forcefully bound, legs and hands."
When a relative and others surged forward to rescue her, guards opened fire and killed a child, witnesses said.
"[Dhuhulow] was asked several times to review her confession but she stressed that she wanted Sharia law and the deserved punishment to apply," local leader Sheikh Hayakallah said.