Japanese Emperor Akihito has made his first public appearance since being diagnosed with prostate cancer.

The 69-year-old emperor attended traditional New Year greetings at the Imperial Palace, waving to tens of thousands of well wishers from a glass-enclosed balcony.

He was joined by other family members, including Empress Michiko and their eldest son, Crown Prince Naruhito.

The palace announced on Saturday that the emperor had prostate cancer and would undergo surgery in about two weeks. Doctors have said the emperor has a good chance of a full recovery.

The emperor will spend about a month in hospital

It was an unusual announcement by the palace, which is strict in protecting the privacy of the imperial family.

"I am very pleased to celebrate the New Year together with you," Emperor Akihito told the crowd.

"At the beginning of the year, I pray for the peace of people in our country and the world."

More than 72,000 people attended the event, in what was believed to be the largest gathering for the annual event since 1994.

Akihito assumed the throne in 1989 following the death of his father, Emperor Hirohito. It was only announced after his death that Hirohito had suffered from cancer.

Emperor Akihito is expected to remain in hospital for a month after he is admitted in mid-January.