A resident called Saturday and alerted them to a suspicious device seen in the 1800 block of Estrella Avenue, police said.
Police responded to the area around 8:20 p.m. The device was found attached to a city sign. Officers determined the device to be suspicious in nature and began evacuating residents nearby, said Sergeant Nick Manfredi of Monrovia Police.
Police would not reveal how the device was attached to the sign, or what type of a device it was.
Roughly 10 to 15 residences were evacuated, Manfredi said. An apartment complex was also cleared, according to scanner reports.
Roads to the immediate area were also closed while bomb technicians with the Los Angeles County Sheriff's bomb squad worked to secure the device. Technicians utilized a robot to retrieve the device, which was eventually detonated, Manfredi said.
It was nearly two hours before the all clear was given and residents were allowed back in their homes, police said.