Witness Geoffrey Kotia, who was supervising children's activities in the upmarket shopping centre, said he was shot by the attackers, accused of killing Somalis and mockingly told to telephone Kenyan President Uhuru Kenyatta for help.
"One of the attackers then said 'You people, we have been giving you chances to become Muslims but you don't, and instead you go ahead and kill our people'," Kotia told the trial of four men who are accused of helping the attackers.
Witnesses from the September attack, which left at least 67 dead including women and children, have previously said that some of the attackers allowed women and children to leave, while others were indiscriminate in their attacks.
The attack was claimed by Somalia's Al-Qaeda-linked Shebab insurgents who are fighting to oust the country's internationally-backed government and African Union troops including Kenyan soldiers.
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The trial in Nairobi, which opened on January 15, has heard evidence from people who were at the mall when the gunmen launched their attack, as well as witnesses involved in transactions related to the purchase of the car that the attackers used to drive to Westgate.
The four accused of providing logistical support -- Adan Mohamed Abidkadir Adan, Mohamed Ahmed Abdi, Liban Abdullah Omar and Hussein Hassan Mustafah -- have all pleaded not guilty to charges of supporting a terrorist group.