Chicago police are warning CTA riders about a string of robberies on the Red Line and Blue Line downtown and on the South Side.

Police said there have been at least seven robberies since Aug. 11 involving a group of two to seven people who approach CTA passengers on board trains and use force to take their belongings.

The robberies happened at the following times and locations:

• Around 3:50 a.m. on Aug. 11 at the Cermak-Chinatown stop on the Red Line.
• Around 4:45 a.m. on Aug. 27 at the 69th Street stop on the Red Line.
• Around 12:30 a.m. on Aug. 29 at the 63rd Street stop on the Red Line.
• Around 1:40 a.m. on Sept. 5 at the Jackson stop on the Red Line.
• Around 1:50 a.m. on Sept. 5 at the Jackson stop on the Blue Line.
• Around 8:40 p.m. on Sept. 9 at the 47th Street stop on the Red Line.
• Around 2:50 a.m. on Sept. 11 at the 47th Street stop on the Red Line.

Police had only a vague description of the robbers, and urged anyone with information on the robberies to contact Area 1 detectives at 312-747-8384 or Area 3 detectives at 312-744-8263.