Japanese Emperor Akihito turned 79 on Sunday, reassuring thousands of well-wishers he has regained his health since heart by-pass surgery, and remembering those affected by the quake-tsunami disaster.

The softly-spoken monarch gave his birthday address from a glass-covered balcony at the Imperial Palace overlooking the East Garden, as crowds braved the bitter cold, waving small Japanese flags.

"In February, I had heart surgery and worried many people. Please remain assured that I am now living normally like before," he said in a brief speech.

Surgeons successfully carried out the four-hour operation on February 18 after tests showed a narrowing of two of his coronary arteries. It was the emperor's first operation since he was treated in 2003 for prostate cancer.

The Japanese throne is held in deep respect by much of the public, despite being stripped of much of its mystique and its quasi-divine status in the aftermath of World War II.

"He represents the people, and it was nice to see him in such good shape," said retiree Takeo Nagahashi, who was among more than 10,000 people who came to the palace to see the emperor's address.

"I was worried and seeing his healthy face made me happy. I hope he will live for a long time."

Another retiree Yuichiro Sato said: "It was a beautiful speech that reassured me. He seems healthy."

During his birthday address, Akihito said his thoughts were with those who have be unable to return to their homes since the devastating earthquake and tsunami that hit northern Japan in March 2011.

"I plan to spend my time praying for the happiness of all of the Japanese public, particularly those who were affected by the disaster," Akihito said, describing the year as a "difficult" one.

The disaster crippled the Fukushima Daiichi nuclear plant, which went through meltdowns and explosions, forcing tens of thousands of residents to abandon their homes, farmland, fishing boats and livestock.

Before his birthday, Akihito told a ceremonial press conference that his health has recovered to the point where he can play tennis.

He said he wanted to continue carrying out his public duties at the same level, despite calls from his family and court officials to reduce his workload.

Since his heart surgery, Akihito and his wife Michiko have travelled around Japan, and in May they visited Britain to celebrate Queen Elizabeth II's Diamond Jubilee, the royal couple's first overseas trip since 2009.