Three people were seriously injured when a hand grenade exploded at a waste collection station in Pudong on Saturday afternoon, Xinmin Evening News reports.

At about 4:40 pm, a worker came across the "strange" device with a string hanging out of its "tail". Out of curiosity, he pulled the string, causing the grenade to explode.

The blast left a pit in the ground six centimetres deep. Three workers were seriously hurt, while another suffered slight injuries.

Colleagues of the workers immediately called police for help, and the injured were transferred to a nearby hospital.

The slightly injured worker has since been discharged from hospital, but the others were still receiving treatment at the time of the report.

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