A former Mr Universe who has just turned 100 said on Sunday that happiness and a life without tensions were the keys to his longevity.

Manohar Aich, who is 4ft 11in (150cm) tall, overcame many hurdles – including grinding poverty and a stint in jail – to succeed as a bodybuilder.

His children, grandchildren and great-grandchildren gathered with him at his home in Kolkata to celebrate his birthday.

Hindu priests chanted prayers while a feast was served to honour Aich, winner of the 1952 Mr Universe title.

Aich rippled his muscles and flashed a toothless grin for his guests, before saying his ability to remain happy during difficult times was the secret to his long life.

He believes his simple diet of milk, fruit, vegetables, rice, lentils and fish has kept him healthy. He does not smoke and has never touched alcohol.

"I never allow any sort of tension to grip me," he said. "I had to struggle to earn money since my young days, but whatever the situation, I remained happy."

Aich, who was born in the small town of Comilla in Bengal, was a puny youngster. But he decided to build his muscles when as a schoolboy he saw a group of wrestlers in action.

In 1942, in British-ruled India, he joined the RAF. It was there that he began his bodybuilding in earnest.

Encouraged by a British officer named Reuben Martin, who introduced him to weight training, Aich earned praise for his physique from his comrades.

Some years later, however, he was thrown into prison when he protested against colonial oppression.

"It was in the jail that I began weight training seriously. This helped me prepare myself for the world championship," said Aich.

"In jail I used to practise on my own, without any equipment, sometimes for 12 hours in a day," he recalled.

The jail authorities were impressed with his perseverance and gave him a special diet to help build his stamina.

India's independence in 1947 led to Aich's release from jail. Dogged by poverty, he and his wife struggled to put their four children through school and there was little cash to indulge his passion for bodybuilding.

In 1950 he won a Mr Hercules contest and that spurred him to set his sights on the Mr Universe tournament in London.

In 1951, he came second in the contest, and stayed on in London to prepare for another shot at the title. He returned to India after becoming Mr Universe in 1952.

Back in India, he won several contests, including the Asian Bodybuilding Championships. He was also affectionately nicknamed Pocket Hercules, due to his short stature.

Six decades later, Aich helps his sons run a gym and fitness centre and advises and guides juvenile bodybuilders.

He was still lifting weights himself at the age of 99, but a minor stroke last year meant he had to stop.

Neither of his sons took up bodybuilding, but many of Aich's pupils have succeeded in the sport – including Premchand Dogra, the 1988 Mr Universe.

Aich says he has few regrets – but he does wish he had got to meet Arnold Schwarzenegger, a fellow Mr Universe winner. Aich is a fan and has seen many of the actor's films. "I like the incredible stunts he does," he said.