Pol O'Geibheannigh was walking back from the cinema when he saw a woman lying in a pool of blood.
He told Sky News: "It was just wall to wall with armed police. You could see a woman on the floor.
"She was lying in a pool of blood with a red blanket on top of her. I went into the bar and it wasn't until an hour later that they put the tent up."
As morning broke officials were seen removing a body bag from the area.
Tommaso Greco, 26, who works in Pret opposite the scene, said: "I am a little bit worried, for sure. Because it could happen to anyone. They were just in the middle of the street. And here, generally, there are a lot of people. It is a tourist area."
He added that staff at the shop had initially been told that it would not open today, but that later changed.
:: Anyone with information about the Russell Square attack is asked to contact police on 101 or call Crimestoppers anonymously on 0800 555 111.