Japan's Emperor Akihito marked his 84th birthday as tens of thousands of well-wishers gathered at the Imperial Palace grounds on Saturday morning.

Local media said 45,900 people had come to the palace - the largest number on record for the current emperor - waving flags and cheering as the emperor and his family made their way onto a balcony.

The birthday appearance is an annual event but on December 1, the government approved his abdication, boosting the number of visitors to the palace seeking to catch a glimpse of Akihito and his successor.

Akihito, who has had heart surgery and treatment for prostate cancer, said in rare remarks last year that he feared age might make him hard for him to fulfil his duties.

He is due to step down on April 30, 2019 after spending much of his nearly three decades on Japan's throne seeking to soothe the wounds of World War Two.

His abdication is the first by a Japanese monarch in about two centuries. He will be succeeded by his heir, 57-year old Crown Prince Naruhito.

Once considered divine, Japan's emperor is defined in the post-war constitution as a "symbol of the state and of the unity of the people", but has no political power.