At about 2 p.m., three unidentified suspects accosted two Alvarado Middle School students as they walked home from school.
The robbery was interrupted by a Union City workers, and one student was slightly injured.
The students said the suspects appeared to be about 17 or 18 years old. Investigators were working with the students to try to identify them.
About the same time, a second robbery occurred near the Burger King restaurant at the intersection of Alvarado Niles Road and Santa Maria Drive.
Six suspects approached a large group of middle school students in their early teens and robbed four of them gunpoint, taking cell phones, money and other personal property.
Responding officers were able to locate and arrest six suspects: four juveniles and two young adults. All were residents of Union City.
Police recovered several pieces of property as well as two realistic replica handguns.
Investigators believe several other victims might have been involved in the incidents. Union City parents are asked to talk to their children and see if they were victimized Thursday.
Anyone with information about the incidents is asked to call the Union City Police Department at (510) 471-1365 or leave an anonymous tip at (510) 675-5207.