He plays a tough cop from a village on the Maharashtra-Goa border in the film.
"We've tried to achieve for Ajay the physique of a man who could bash up 20 people, if he so wished. It's not a six pack look. Anyone can achieve that with injections and steroids," said Shetty.
"'Singham' is a raw action film filled with hand-to-hand combat scenes, something one rarely gets to see in movies in this day and age of high gadgetry. Ajay's trainer Prashant Sawant is putting him through the paces," he added.
Talking about Ajay's fitness regime, his trainer said: "We trained for three months. Ajay plays a very tough cop. He needed to look the part by losing weight, developing more cuts on his body and appear more lean and muscular.
"After training for an-hour-and-a-half, I oversaw his diet and made sure he maintained a high-protein low-carb diet every three hours. Then I had him do a rigorous combination of weights and circuit super sets that incorporated all techniques including push-ups and pull-ups."
Sawant says it wasn't too tough for Ajay to build his body.
"It didn't take him much time. Ajay has a great body and he has always been a fitness freak. It's not difficult for him to maintain it."
The movie is being produced by Reliance Entertainment.