A group of about 200 migrants rampaged through a traditionally Roma section of the south-eastern Spanish town of Roquetas de Mar seeking the murderer of a man from Guinea-Bissau.
A police report said on Saturday in Madrid that the 41-year-old man was stabbed to death during a fight on Friday.
Police said they had no suspects, which apparently prompted other asylum seekers to head to the Roma neighbourhood in search of a suspect.
Police said the crowd then went on to set cars on fire and throw flaming objects at policemen.
It said they had to eventually call in reinforcements before they got the area under control.
Police said no arrests were made, but the search for the stabber continues.