Additional Sessions Judge H B Raval convicted the 12 persons and sentenced them to life imprisonment, and also slapped a fine of Rs 1 lakh on each convict.
Four others were convicted to six months' imprisonment alongwith a fine of Rs 10,000 each.
Among those convicted is Banubhai Bharwad, the sitting BJP councillor in Nadiad municipality.
The case dates back to 2002, when one of the accused - Sahid Natubhai - hatched a plan to kidnap Usman Vora, with whom he used to jointly run a cold-storage, following a dispute over mismatch of financial account of their business establishment at Nadiad town in Kheda district.
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According to police, Natubhai got in touch with one Navdhanbhai Bharwad and asked him to make sure that Vora paid up the money that he owed him. Navdhanbhai then approached Bhanubhai Bharwad to settle the case.
Navdhanbhai, Bhanubhai and others then kidnapped the complainant on July 8, 2002. They locked him up and threatened to kill him.
However, Vora was later released by them.
Bhanubhai was not a BJP councillor at the time of the incident.
Later, the accused were booked under IPC sections 147, 148, 364 (A), 120 (B), 507 (2), 342 and 447.
The court today found the rest 16 guilty, and awarded life sentence to 12 of them and six months' imprisonment to four others.