Former US President Bill Clinton said: "I was saddened to learn of the death of Lady Margaret Thatcher. The United Kingdom has lost its first woman Prime Minister, an iconic stateswoman, and a fearless leader.
"The United States has lost one of its dearest friends and most valued allies.
"Lady Thatcher understood that the special relationship which has long united our two nations is an indispensable foundation for peace and prosperity. Our strong partnership today is part of her legacy.
"Like so many others, I respected the conviction and self-determination she displayed throughout her remarkable life as she broke barriers, defied expectations, and led her country.
"Hillary, Chelsea, and I extend our condolences to her family and to the people of the United Kingdom."
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His mother Princess Diana was killed in a car crash in Paris in 1997 when he was just 12 and his brother William was 15.
The 21-year-old, who was arrested on Sunday, also allegedly injured two other people in the attack in the German city of Reutlingen.
She was going to the aid of a family member who got out of the car in the middle of the park and was dragged away by a different tiger.