Travelling criminal gangs are being blamed for the theft of 47 mobile phones from people in bars and clubs over the past three weekends.
At least two gangs have targeted busy city centre venues.
Police urged people heading for the city centre this weekend to keep their phones secure at all times, ideally in a zipped-up bag or inside pocket.
The message was echoed by bar owners and security staff.
Police raised the alarm three weeks ago when it emerged 18 phones had been stolen at licensed premises in one weekend.
The following weekend, 16 phones were taken and last weekend 13 were stolen.
Officers said they would normally expect four or five such thefts at city centre venues in the course of a weekend.
City centre police commander Inspector Chris Cockerill said there had been a similar spate of thefts last summer.
"Last year, we had arrests in places such as Sheffield and London where officers stopped vehicles and found bags of phones which had been stolen in Leicester," he said.
"The people who are committing these thefts are organised and they travel to different towns and cities. They are looking for dark, busy venues where it's generally a younger crowd.
"They are very proficient at what they do and it only takes them a split second to take a phone, a wallet or a purse if given the opportunity."
Last weekend, officers arrested four men and a woman.
Three of the arrests followed a tip-off from CCTV operators, while two men were detained when security staff at Walkabout bar, in Granby Street, called police after spotting them behaving suspiciously.
A number of phones and SIM cards were recovered.
The five, who were from the West Midlands and Northampton, have been released on bail.
Chris Brannigan, general manager at Walkabout, said: "Phone theft can happen in any bar, but we're determined it's not going to happen here.
"Our bar and security teams are on the lookout for suspicious behaviour and we'll have no hesitation in calling the police if we spot anything.
"We would also urge people to be careful with their belongings when they're out this weekend, wherever they are, and if they see anything suspicious to report it to us or the police."
Citywatch, of which many city pubs and clubs are members, is also working with police.
Mark Williamson, of Citywatch, said: "We have a network of 60 bars and clubs linked to each other and to the city's CCTV room.
"If any of our members spots thieves at work, we will report it immediately and there is a good chance they will be caught on video."
Police urged people to download GPS tracking programmes for their phones.
The technology enables police to trace the exact location of stolen handsets.
Crime-prevention advice is available at a Safer Leicester Partnership website at: