South Wales Police say they are stepping up patrols on the Taff Trail through Cardiff city centre, after a spate of robberies.
Officers say there have been six incidents in the Sophia Gardens and Pontacanna Fields area over the last two weeks.
The latest attack was on Friday when a man was robbed by three males.
Detectives said the incidents may be related, and are urging public to be vigilant in the area.
Det Insp Chris Cullen, from Fairwater CID, said: "While Cardiff remains a safe place we would ask people to be aware of their personal safety when they are walking alone at night and to be mindful of their surroundings.
"I would ask people to follow normal crime prevention advice such as not having valuables on show, to stick to well-lit areas, and if possible to walk with company in the hours of darkness."
In the last incident, a 52-year-old man was assaulted and his wallet, mobile phone and watch were stolen.
Det Insp Cullen added: "Some of the witnesses have given similar descriptions so it is possible that the same people are responsible for these incidents particularly because of the similarities of the locations, times, and the nature of the offence.
"We will not tolerate people, who are simply walking home from work or a night out becoming victims of crime in this way.
"Rest assured we are increasing our patrols in these areas and would appeal for anyone with information on who is responsible to contact us."
Anyone with information is asked to contact Fairwater CID on 02920 527248 or Crimestoppers anonymously on 0800 555 111.