The Melbourne woman was also placed on lockdown on the Tube as three knifemen rampaged in London Bridge and was in Notre Dame the day a police officer was stabbed outside the famous cathedral.
Ms Monaco told of "absolute pandemonium" as she was locked in a shop in Placa de Catalyuna, at the top of Las Ramblas, after terrorists mowed down pedestrians in the busy street on Thursday.
“All of a sudden we were locked in the shop and confusion started to sprinkle among the people,” she told Australian radio station 3AW.
"The next second we knew we were literally running for our lives to the very back of the store where we were told to get on the ground away from the windows and lay as flat as possible."
She told 9 News: "We didn't see the van, we could just see the crowds and the police and some people who had clearly seen something terrible happen.
"We were looking at about 200 people crowded out in the square.
"Suddenly everyone outside just started screaming and everyone turned at the same time.
Spain terror suspects: What we know so far
"People were literally pushing to get into the store off the street... We were genuinely terrified."
Ms Monaco described children screaming amid "considerable panic" in the shop before they were evacuated by police through a side door.
“I feel a bit rattled, it’s not how I really expected today to go,” she added.
“But I don't feel like I want to go home. I feel like I want to stay here and not let them, whoever they might be, win."
A message released through the jihadi group's news agency Amaq: "The perpetrators of the attack in Barcelona are Islamic State soldiers and carried out the operation on command of Khilafah of targeting coalition countries"
Spanish police have arrested three suspects, including 28-year-old Driss Oukabir. They are still hunting for the driver of the van.
Armed officers also shot dead five suspected terrorists wearing fake suicide vests as they attempted to carry out a second attack in the seaside resort of Cambrils, 70 miles from Barcelona, in the early hours of Friday morning.
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