Cerys Lowe, 21, said her "heart was racing" when four males, described as in their mid-late teens, burst into Carphone Warehouse at Forge Retail Park in Telford, threatening her and boss Christopher Watkins.
She told Shrewsbury Crown Court how she reached for the panic button, but one of the quartet shouted at her to stop or she would be knifed. The incident happened on November 10 last year.
Ms Lowe was giving evidence at the trial of two boys, one aged 15 and a 16-year-old, for robbery. The 15-year-old was joined in the dock by an interpreter. The other boy has not been in attendance.
She told how one of the group went straight to the counter to watch her and Mr Watkins, while the accomplices went for goods and guarded the front doors.
"My hands were shaking and my heart was racing," she said. "I was very nervous.
"He walked towards us. He was there to watch us, I assume in case we tried to call the police.
"We have panic buttons underneath the tills. I leant down to try and find one but he got very angry and started shouting and screaming at me. He said he would stab me if I moved again. He was very loud and aggressive."
A pink iPhone SE worth £239, two Samsung tablets and a number of phone cases were taken.
Shopper Adrian Davies was in the store with his wife and son as the drama unfolded.
He said: "One of them was very aggressive towards the staff. He said 'I've got a knife, I'm going to stab you.' I didn't see a knife but he kept putting his hand in his pocket."
The two boys had both offered guilty pleas to theft but deny robbery, which requires violence or the threat of it. These pleas were not accepted.
Prosecutor Steven Bailey said in opening the case that the gang, none of whom live in the area, travelled to Telford in an Audi A3 with the intention of robbing the store. The shop's address was found in driver Maxim Voicu's sat nav, and he was arrested.
Voicu, who along with another teenager and another man, has pleaded guilty to robbery and awaits sentencing, stayed in the vehicle and parked deliberately away from CCTV cameras, while the group entered the shop.
CCTV of the four men entering the shop together and all running out within a minute was played to the jury.
The trial continues.