Seven months after he was one of the first responders to Melbourne's Bourke Street tragedy, firefighter Graeme O'Sullivan has seen first hand the deadly van rampage in Barcelona.

Metropolitan Fire Brigade commander Graeme O'Sullivan is holidaying with wife Kim in Barcelona and was at a rooftop bar when he heard a vehicle being driven through the popular Las Ramblas tourist boulevard below.

"We were about 20 metres away, on the hotel's sixth floor terrace relaxing at the time," he told Fairfax Media.

"I heard a rumbling sound to begin with, then a distinct thud sound of the vehicle hitting pedestrians and some pretty horrific screams.

"A short time after that, [we] heard some shots fired."

Mr O'Sullivan said the two men who carried out the attack got out of the van and started firing shots into the crowd and into a nearby shop before fleeing on foot.

"We went over to the edge of the roof...where we could see the vehicle," he said. "It was a white van, heavily damaged at the front with the front bumper on the ground."

"We could also see one of the victims, a male, lying on the road bloodied."

"There were about 40 people up on the rooftop terrace. We all rushed over to see what it was."

He said the Las Ramblas area was popular with tourists and was packed with thousands of summer holidaymakers of all ages.

"Pretty horrific scenes here this afternoon," he said."

Mr O'Sullivan said he and wife, who is a fire station officer and former nurse, wanted to rush downstairs to help injured people on the street.

"Kim was desperate to get downstairs and into the street to help people," he said.

"I had to grab hold of her and restrain her from going outside because it wasn't safe."

He said the scene was "a bit too close to home", and brought back his memories of being a first responder to the Bourke Street attack in Melbourne.

"We couldn't believe it, initially. It was a little bit too close to home. It was very similar in a lot of ways to the Bourke Street incident back in January," he told radio station 3AW.

Six people were killed, including a three-month-old boy and 10-year-old girl, and dozens more injured when a car mowed down pedestrians during the busy lunchtime rush in Bourke Street during the school holidays.

Mr O'Sullivan arrived on Bourke Street with 10 minutes of the attack and was among the emergency teams providing first aid to those affected.

"Clearly this time there were links with terrorism, and Bourke Street wasn't that, but the result is still the same," he said.

"It's challenging for us because our natural instinct is to jump in and help, but we were unable to do that today."

Mr O'Sullivan said the attack in Barcelona, like Bourke Street, occurred in an area crowded with families enjoying the festive atmosphere.

"Over here at the moment it's summer holidays ... there's lots of large crowds of families of all ages, thousands of people, and everyone is just enjoying the festive time of year," he told 3AW.

"It's very sad that another one of these incidents has occurred."

Victorian Police Minister Lisa Neville said extra police would be deployed across Melbourne on Friday to provide reassurance.

"It goes to why we need to continue to make our city safer, to have in place some of those temporary bollards and then the permanent bollards because these acts can change the nature of your city," Ms Neville said.

Another Australian tourist, Susan McLean, also said the horror that unfolded moments after a van ploughed into crowds reminded her of the Bourke Street tragedy.

Ms McLean was about 100 metres from the scene as the van zigzagged down one of the city's busiest avenues, mowing down pedestrians and leaving bodies strewn across the ground.

"All of a sudden there was this tidal wave of people running from both Placa de Catalunya and Las Ramblas towards us screaming, crying and with absolute terror etched on their faces," she told Channel Nine.

"Initially we couldn't hear anything other than all these people running towards us. Several of them were calling 'gun, gun', so first of all we thought someone had been shot.

"Then they just kept sort of - it was all in Spanish ... it was very difficult to understand but they were sort of pushing us into shops."

Ms McLean, who was separated from her husband in the panic, said the scene reminded her of the Bourke Street tragedy.

"That was quite scary because I wasn't sure where he was but the shopkeepers were great. They pulled everyone inside and put the shutters down," she said.

"My first reaction was the Bourke Street [rampage], because that is what it reminded me of, the vision of people fleeing in just such terror."

- with AAP