Surgeons started operating on King Harald at 8:30am Friday and said they finished a little over three hours later, at around 11:45am. The king was said to be in stable condition when palace officials and doctors briefed reporters late Friday afternoon.

Doctors said the operation proceeded with no complications.

The monarch's heart was stopped for nearly an hour, while surgeons replaced his defective valve with a what was described as a "biologic" one. Dr Odd Geiran, a medical professor who led the surgical team, declined to say from where the valve originated.

He said the team also performed a bypass procedure on the monarch, who smoked for years but quit last year following a bout with bladder cancer.

King Harald is expected to remain in the hospital (Rikshospitalet) for the next two weeks. Then he's expected to remain on sick leave through May, probably at the timber lodge in the hills above Oslo called Kongsseteren.