Bricks and bottles were thrown at police as they tried to control a large disturbance in Halifax which has left a man critically ill in hospital.
Officers were called to the Pellon area of the town on Wednesday night where they found a number of people injured and a "hostile crowd".
A 27-year-old man is in a critical condition in hospital. Two men aged 39 and 44 suffered less serious injuries.
Four men, aged 23, 24, 36 and 38, were arrested and are in police custody.
A West Yorkshire Police spokesman refused to comment on claims from residents that a feud between two families in the area had sparked the incident.
He said two other men arrested for public order offences were issued with fixed penalty notices and released.
When officers tried to split up the crowd bricks and bottles were thrown at them. After the situation was brought under control officers stayed in the area to ensure calm, the spokesman added.
Ch Insp Steve Cotter, of Calderdale's Community Safety unit, said: "High visibility patrols were deployed in the area last night to provide reassurance to residents and neighbourhood policing team officers are in the area again today to continue that work.
"We are also meeting with residents and community leaders again today to reassure them that this was an isolated incident.
"Incidents such as this are thankfully very, very rare in Calderdale."