In a statement addressed to King Charles, Queen Margrethe said Queen Elizabeth had been a "towering figure among the European monarchs and a great inspiration to us all".
King Harald sent his condolences to the royal family and the British people.
"For nearly a century, Her Majesty devoted her life to the service of the Commonwealth, following the British people through good days and bad, in times of happiness and sorrow," the 85-year-old monarch said in a statement.
Sweden's King Carl Gustaf, 76, said in a statement he was "deeply saddened" by the news.
Queen Elizabeth, Britain's longest-reigning monarch, the nation's figurehead and a towering presence on the world stage for seven decades, died peacefully at her home in Scotland on Thursday.