Polish President Karol Nawrocki sent condolences to the Norwegian royal family and people, saying he had learned of the king's death "with profound sorrow" and was praying for them in this difficult time.
Harald had reigned since 1991, succeeding his father Olav V, making him Europe's longest-serving monarch.
Under Norway's hereditary succession rules, the throne passes automatically to his eldest child, Crown Prince Haakon, who becomes king immediately on his father's death.
As Harald's health worsened this week, crowds gathered outside the Royal Palace in Oslo, with thousands singing the national anthem on Thursday evening.
The BBC reported that after 35 years on the throne, the late king will be remembered as a popular, reforming monarch who modernised the institution, broke with royal tradition by marrying a commoner, and became a unifying figure during Norway's hardest moments.
(ał)
Source: PAP, BBC