Mattarella wrote to German President Frank-Walter Steinmeier saying "I have learned with deep sorrow of the passing of Helmut Kohl, a political personality of extraordinary significance in the recent history of Germany and the whole of Europe".
"Working closely with my very good friend to help achieve a peaceful end to the Cold War and the unification of Germany within North Atlantic Treaty Organisation will remain one of the great joys of my life", Bush said.
Added Harris: "He was also a cherished friend of the Jewish people and Israel, and frequently spoke about his desire to restore a Jewish presence in his country after the devastation wrought by the Nazi Final Solution".
Once viewed as a provincial bumbler, Kohl combined an understanding of the worries of ordinary Germans with a hunger for power, getting elected four times.
He went on to become the Minister-President of Rhineland-Palatinate from 1969 to 1976. As a 15-year-old, he was about to be pressed into service in an anti-aircraft gun unit when the war ended.
"Helmut Kohl filled the European house with life - not only because he built bridges to the west as well as to the east, but also because he never ceased to design even better blueprints for the future of Europe", Juncker said.
He helped a Germany that was split during the Cold War between a capitalist west and a communist east make the traumatic transition to a unified democracy.
The Kremlin said Russian President Vladimir Putin had sent condolences to Germany's president and to Merkel and cited him as saying Kohl "will be remembered in Russia as a resolute supporter of friendly relations between our countries".
Of overwhelming character and physique, Helmut Kohl - who according to his sons sacrificed the family for politics - then retired from public life.
Kohl's legacy includes the common euro currency that bound Europe more closely together than ever before.
Gentiloni, for his part, tweeted: "Honour to the memory of Chancellor Kohl". His wife, Hannelore, killed herself in July 2001 after suffering from a rare skin condition and depression.