City Police Chief ACP Jauteh Dikun said preliminary investigations showed the 19-year-old student had stopped to straighten her headscarf when a man suddenly snatched her bag from behind before attempting to flee on a waiting motorcycle.
He said the woman shouted for help, prompting a Honda vehicle driver to stop the culprits from fleeing by blocking their path in the 3.30pm incident.
However, another woman motorcyclist failed to stop her motorcycle from ramming into the Honda, which caused her to fall.
She was taken to the Sabah Women and Children's Hospital Likas for treatment.
The culprits, who had no documents, were handed over to a police team who arrived later.
Photos and videos of the incident, which went viral over the social media, showed the duo being kicked and punched by a group of civilians as they waited for the police team to arrive.