Mpumalanga police have freed 21 people who have been held hostage for over five hours inside an Ackermans store in Standerton on Sunday.

Captain Busaphi Sibanyoni said three armed men entered the store on Sunday morning and ordered staff and customers into a storeroom where they were held hostage.

"The police were alerted to the robbery. But when they tried to enter the store from the back a robber started shooting randomly and wounded a 23-year-old police reservist constable on the leg. He was rushed to Standerton Hospital," he said.

The three men aged 22, 24 and 28 have since been arrested and charged with armed robbery with aggravating circumstances.

They are expected to appear in the Standerton Magistrate's court on Monday.