The late-afternoon attack in the heart of Barcelona was swift, unexpected, and devastating.
Travelling at a high speed, a white van mounted the footpath and zig-zagged through pedestrians in the popular Las Ramblas district.
It travelled several hundred metres, before finally coming to a halt in the thoroughfare.
Witnesses describe hearing people screaming and seeing them running for their lives as the van driver fled on foot.
Australian tourist Julia Monaco was in a nearby store as the incident unfolded.
Casualties were scattered over the streets, with scores of heavily armed officers on the scene within minutes.
Buildings were cleared and train stations shut down as authorities searched for suspects.
But Australian tourist and cybersecurity expert Susan McLean who hid in a shop in the aftermath, says police were clearly caught off-guard.
Terrorist group I-S has claimed responsibility for the attack, but that has not yet been verified by appropriate authorities.
British security expert Dr Sajjan Gohel says there is a difference between people inspired by I-S ideology, and those taking instructions directly from them.
There has been universal condemnation from world leaders.
German Chancellor Angela Merkel described the attack as "revolting", while British Prime Minister Theresa May says she is sickened by the senseless loss.
Prime Minister Malcolm Turnbull says Australia remains "resolute" with Spain, a sentiment echoed by the French and Canadian leaders.
U-S Secretary of State Rex Tillerson also offered a warning to attackers.
