Police are warning San Mateo bikers and residents to be on guard following a string of strong-arm robberies carried out in the last five days.Five separate incidents of strong-arm robberies have been reported since Saturday within a four-block radius of South Delaware and Cypress streets, according to the Police Department. The similarity of the attacks, each of which took place between the hours of 7 p.m. and 10 p.m., leads investigators to believe they are related. In each case, a Hispanic man had been walking or bicycling through the area and was accosted by a group of black men wearing hoodies. Some cases involved physical assaults or restraining the victim, all cases involved cash and property being stolen.Police reported that the victims were unable to provide detailed suspect descriptions, and are asking the public to assist the investigation by providing any information to the San Mateo Police Department investigations unit at (650) 522-7650, or through the secret witness line at (650) 522-7676.In response to the robberies, police reported Wednesday that plain clothes surveillance and high profile patrols of the area have been increased. Neighborhood Watch groups and neighboring residents have been notified and encouraged to be particularly aware of their surroundings and to report suspicious activity immediately.