The Foreign Office is helping the family of a woman who was beheaded in a shop on the Spanish island of Tenerife.
Spanish media said a 28-year-old homeless Bulgarian man had been arrested in the town of Los Cristianos.
Witnesses said a man entered a Chinese supermarket in a shopping centre and stabbed then beheaded the 62-year-old woman. He is said to have dropped the head outside before being tackled.
A local official said the man appeared to choose his victim at random.
Local officials have been analysing CCTV footage of the attack which shows a man walking into the supermarket - which sells Chinese food and tourist souvenirs.
Witnesses said the man attacked the woman without saying a word.
"Apparently this gentleman without any motive or any reason... entered the shop and then cut this woman's neck and took the head in his hand outside," said local councillor Manuel Reveron.
A security guard then managed to wrestle the man to the ground, he said.
"I parked my car and saw a man running out with something bloody in his hands and a security guard chasing him," one witness was quoted as telling local radio.
"He threw it to the ground, it almost hit me and what he had been carrying was a woman's head."
In a video posted on YouTube, Colin Kirby of Tenerifemagazine.com said security guards held down the suspect until the police arrived.
"The security and the police had to hold people off - they were queuing up - they were trying basically to kick the hell out of the guy," he said.
Christina Perez, a legal representative at a nearby court, said she and her colleagues ran indoors for safety.
"Everybody is shocked. It's a very safe area. You can usually go anywhere you want in the day or at night. This is really not normal."
Police sources told Spanish media the suspect had a police record.
Dominica Fernandez, of the Regional Interior Ministry, said the attack appeared to be random and that the suspect was well known in the area.
Regional newspaper La Opinion said the suspect had received treatment at the psychiatric unit of a local hospital in February after being involved in previous violent incidents.
The BBC's Maddy Savage said this kind of violence was extremely rare in the Canary Islands which attract more than 10 million tourists each year.
A Foreign Office spokesman said: "We are in touch with the next of kin and are providing consular assistance.
"Consular officials in Tenerife are in contact with local authorities about this tragic incident and our condolences go out to the family at this difficult time."