Detectives in Bradford are investigating whether four robberies carried out in the city since Friday are linked.
On each occasion a group allegedly attacked and stole mobile phones and wallets from men who were walking alone in the city.
A 26-year-old man was taken to hospital after receiving face, head and arm injuries in one attack in Willowfield Street at 1900 GMT on Sunday.
He was said to be in a comfortable condition at Bradford Royal Infirmary.
Arrests made
The robbery followed other incidents in Summerville Road on Friday and Woodhead Road on Saturday evening.
Police say two men, aged 18 and 27, have been arrested in connection with the incident in Willowfield Street and are being held in custody.
In a fourth incident on Sunday, a 23-year-old man was assaulted by the occupants of a black car in Laisteridge Lane at about 1955 GMT.
Detective Inspector John Armitage, of West Yorkshire Police, said: "At this stage we are keeping an open mind, but are investigating whether there is anything that may link these offences."
Police say the robberies were carried out by groups of between two and eight Asian men.