The blood-stained cassock worn by the late pope John Paul II on the day of a failed assassination bid on May 13, 1981 at the Vatican went on display Wednesday in the Polish city of Krakow.

Karol Wojtyla, the only Polish pontiff in history, was seriously wounded in the shooting attack by Turkish extremist Mehmet Ali Agca in St. Peter's Square.

Thirty-four years later, the reasons behind the attempted assassination are still a mystery.

"From the stains you can tell that John Paul II was bleeding heavily ... In hospital there were moments when physicians had doubts whether he could be saved," Cardinal Stanislaw Dziwisz, Archbishop of Krakow, told Polish media Wednesday.

Dziwisz, former secretary to the pope, was standing next to John Paul at the time of the attack.

Nuns caring for the pontiff kept the long, cream-coloured garment bearing a bullet hole and large blood stains after removing it from the hospital, where he was taken for emergency treatment.

The wildly popular pope, who died in 2005 at the age of 85, was canonised last year.