The witness was giving evidence at the trial of men accused of taking part in a robbery in which Rolex watches worth £400,000 were taken.
Alan Mitchell, 34, and Gary Mitchell, 20, deny robbing Eric N Smith Jewellery in Ayr Road, Newton Mearns, on April 13.
Both men have lodged special defences of alibi.
At the High Court in Glasgow, Moira Thomson, 51, told prosecutor Sheena Fraser that she saw three men coming into the shop and added: "I was aware of something being thrown down to hold the door open. It was a concrete block.
"They were shouting: 'Don't move, stay where you are," two of them went to the Rolex windows and began smashing them with a hammer. They started scooping the watches out the windows."
Mrs Thomson added: "There was a person standing in front of me.
"He had a machete and was telling me to stand still. He was just shouting and waving it. I felt threatened. I felt numb."
She was asked if she could identify any of the men and replied: "All I could see was the weapon. I've totally blocked the rest of it out."
The trial before judge Lord Jones continues.