The knifeman who stabbed a woman to death in a Barcelona suburb on Saturday shouted ‘Allah' before targeting his young victim, eyewitnesses claimed.
Videos showed the killer prowling the street and brandishing the weapon he used to attack the woman.
Spanish police have refused to confirm the suspect’s nationality but he has been described locally as Moroccan.
Spanish media outlets El Mundo and Telecinco, citing witnesses who saw him acting 'nervously' before the stabbing, said he had yelled out the name of Allah before the attack.
Jose Luis, a 58-year-old man who was injured after confronting the knifeman during the fatal stabbing, said: 'I heard some shouting and ran up some steps to the area where the noise was coming from.
'I saw what appeared to be the man going for his victim and yelled at him.
'He turned round and came towards me and tried to stab me at the top of the stairs but didn’t manage to.
'That’s when I tried to hit him. He didn’t like that and picked up some stones from the ground. He threw and hit me with them in the arm, and then attempted to stab me three or four times again,' Luis said.
The knifeman who stabbed a woman to death in a Barcelona suburb on Saturday reportedly shouted ‘Allah' before targeting his young victim
The suspected killer, who was wearing jeans and a hoodie and carrying a large black rucksack, was arrested around midday Saturday in Barcelona's Les Corts neighbourhood
Police are still trying to establish whether the attacker knew his victim, whose age and nationality have not yet been revealed
'But it was easier for him to stab a young woman than a man. The neighbours started shouting "Police, police", and he turned and ran,' Luis continued.
Locals in Esplugues de Llobregat near Barcelona where the 11am stabbing occurred on Saturday say they had seen the suspect in the area in the days before the incident.
One said it looked like he had been 'casing it out'.
Some reports on social media have claimed the stab victim was only 16, but police sources confirmed she was an adult and not a minor, although her age has not been made public.
Detectives are still trying to establish whether the attacker knew his victim, but they have said there was no romantic relationship between the two they were aware of.
Eyewitnesses claimed the knifeman had been invoking the name of Allah before the attack, and that it appeared to be a random stabbing.
The woman, described locally as 'young' and in one unconfirmed report as being of possible Asian origin, was knifed in the throat, chest and stomach.
The brutal killing occurred in a residential area near Sant Joan Deu Hospital.
Esplugues de Llobregat, which is near to Barcelona’s Nou Camp stadium, is home to former footballers Gerard Pique and Dani Alves.
Detectives are still trying to establish whether the attacker knew his victim, but they have said there was no romantic relationship between the two they were aware of
The suspected killer, who was wearing jeans and a hoodie and carrying a large black rucksack, was arrested around midday on Saturday in Barcelona’s Les Corts neighbourhood.
Esplugues de Llobregat Town Hall said in an initial statement: 'We deeply regret the violent death of a person in the town and extend our condolences to their family and loved ones.
'The council condemns all forms of violence and expresses its concern over these serious incidents, which affect community life and public safety.
'The alleged perpetrator has already been arrested by the Mossos d’Esquadra regional police force.
'From the outset, the council has been coordinating with the Mossos d’Esquadra, the force leading the investigation, and the Esplugues Local Police, and is offering all necessary institutional cooperation to help clarify the facts.'
In a second statement it confirmed it had decreed two days of official mourning which started at midnight yesterday.
It described the victim as a local woman.
The Mossos D’Esquadra regional police force said in its own statement in the aftermath of the fatal stabbing: 'Officers have arrested a man as the suspected author of a violent death committed this afternoon in Esplugues de Llobregat in the province of Barcelona.
'At around 11am the 112 emergency number received a report that a man had injured a woman and fled the scene.
'Police and the Emergency Medical Service (SEM) attended the scene, but were unable to save her life.
'The alleged perpetrator was later located and arrested.'
Footage from the scene shows a forensic tent erected, concealing the body from members of the public.
It was initially reported terrorism had been ruled out as a motive although police declined to comment further, saying the incident remained under investigation.