Police are appealing for information after two robberies in the town centre.
A 20-year-old man was robbed of his Sony Xperia phone and wallet under the Park Street Viaduct on Friday.
A man, aged about 17 years old and white, approached the victim from behind and demanded his property.
He was with another man who was aged about 16 years old.
In a separate incident in Guildford Street on Sunday, a 29-year-old man was approached by a man who asked for some change at about 8.25pm.
The victim noticed the man was holding a knife and then the robber felt his pockets and demanded he hand over his wallet which contained cash.
The offender is described as a black man, approximately 23 years old, 5’11” tall with an average build. He wore a black hooded top with the hood pulled up and dark jeans.
Detectives from the South Bedfordshire Robbery Unit are keen to trace anyone who witnessed either of these offences or anyone who can help identify those responsible.
Anyone with information relating to this investigation can contact Det Con Tom Hamm or Det Con Jo Bosson, in confidence, at Luton Police Station, on 01582 394441, Bedfordshire Police on 101, or text information to 07786 200011.
Alternatively, contact the independent crime fighting charity Crimestoppers, anonymously, on 0800 555 111.