A combing operation launched by the police after the explosion on Monday night led to the recovery of 10 similar devices in the area.
Mahesh (24) found a tiffin box lying near the bridge when he went there to ease himself. After his attempts to open the box failed, he flung it on the ground following which it exploded, injuring him, the police said.
Subsequently, he found a similar box which he threw into the nearby Vaigai river and then informed the police who recovered 10 more tiffin box bombs and requisitioned the services of a bomb disposal squad to defuse them.
One of the bombs was found to be "quite powerful," a police official said.
Police teams aided by sniffer dogs are still searching the area and all check-posts have been alerted as the bridge is located close to Home Minister P Chidambaram's Sivaganga parliamentary constituency and is located en route to Rameswaram island.
An official said such bombs are used in group clashes in south Tamil Nadu, including Tirunelveli and Ramanathapuram.