Surgeons said the operation was a success, and there was no sign the cancer had spread to other organs.

"The operation went smoothly as scheduled... and finished successfully," said Ichiro Kanazawa, medical supervisor to the Imperial Household Agency.

Akihito is trying to modernise the monarchy

"His Majesty woke up from the anaesthetic and had a talk with the empress and Princess Sayako."

The empress and her daughter had waited at the University of Tokyo hospital throughout the operation, which lasted three hours and 40 minutes.

More than 6,000 people signed books outside the palace on Saturday morning, wishing the emperor a speedy recovery.

He is expected to remain in hospital for a month. During this time, his constitutional duties have been assumed by Crown Prince Naruhito.

Break with tradition

The news that the emperor was suffering from prostate cancer was given at the end of last year.

The announcement marked a break with tradition for the highly secretive imperial household.

The previous emperor died in 1989 before it was revealed that he too had been suffering from cancer.

Emperor Akihito was treated in a public hospital - the first time a Japanese emperor has undergone surgery outside the palace grounds.

These initiatives are being seen as part of a modest drive to modernise the world's longest reigning monarchy.

Now 15 years into his reign, Emperor Akihito is considered to have more liberal and forward-thinking views than his father, the wartime emperor Hirohito.