Vladimir Putin, the Russian president, sent a telegram to Raul Castro.
The unusually impassioned communiqué saw Putin hail Castro as a "symbol of a whole era of modern world history" saying he was "a wise and strong person" who was "an inspiring example for all countries and peoples" and a "sincere and reliable friend of Russia."
Russia's prime minister, Dimitry Medvedev, said, in part, "Without exaggeration, a whole era of history is gone with Fidel Castro."
Pope Francis sent a telegram to Raul Castro as well, writing, "Upon receiving the sad news of the passing of your beloved brother, the honorable Fidel Castro Ruz, former president of the state council and the government of the Republic of Cuba, I express my sadness to your excellency and all family members of the deceased dignitary, as well as the government and the people in that beloved nation. At the same time, I offer my prayers for his eternal rest, and I entrust the Cuban people to the maternal intercession of Our Lady of La Caridad del Cobre, patroness of that country."
Rafael Correa, president of Ecuador, tweeted, "He was a big one. Fidel dead. Long live Cuba! Long Live Latin America!"
Enrique Peña Nieto, president of Mexico, tweeted, "I lament the death of Fidel Castro Ruz, leader of the Cuban Revolution and the emblematic reference of the 20th century ... Fidel Castro was a friend of Mexico, a promoter of a bilateral relationship based in respect, dialogue, and solidarity."
Nicolás Maduro, the Venezuelan president, fired off a tweet storm, writing, "To all the revolutionaries of the world, we have to continue with his legacy and his flag of independence, of socialism, of homeland."
Maduro continued, "He made history together with the peoples of the world to signal a way of dignity...Great history with the principles of Bolivar and Martí," and "Fidel and Chávez constructed el ALBA [the Bolivarian Alliance for the Peoples of Our America], PetroCaribe and left the path of liberation of our peoples...the history absolved."
Maduro added, "I just talked with President Raul Castro to send the solidarity and love to the people of Cuba before the passing of Commander Fidel Castro."
Salvador Sánchez, the president of El Salvador, tweeted, "With deep pain we receive the news of the death of dear friend and eternal companion, Commander Fidel Castro Ruz...Fidel always lived in the hearts of the people of solidarity who fight for justice, dignity and fraternity...Our condolences and solidarity with the brother government and people of Cuba.
Michelle Bachelet, the Chilean president, tweeted: "My condolences to the President Raul Castro by the death of Fidel, a leader by the dignity and the justice social in Cuba and America Latina."
In Brazil, former president Dilma Rousseff, said in a statement: "Fidel was one of the most important contemporary political and a visionary who believed in building a fraternal and just society, free from hunger and exploitation, a Latin America united and strong...My deepest feelings to the Castro family, the children and grandchildren of Castro, his brother Raul and the Cuban people. My sympathy and affection in this moment of pain and farewell."
Mariano Rajoy, the Spanish prime minister, tweeted, "My condolences to the Government and Cuban authorities on the passing of former President Fidel Castro, a figure of historical significance."
Francois Hollande, the president of France, issued a statement, saying in part: "Fidel Castro was a major figure of the XXth century. He embodied the Cuban revolution, in the hopes it aroused and the disappointments it provoked. Actor of the Cold War, he represented a period of history that ended with the fall of the Soviet Union. He represented, for Cubans, pride in rejecting external domination."
Canadian Prime Minister Justin Trudeau said in a statement, "Fidel Castro was a larger than life leader who served his people for almost half a century. A legendary revolutionary and orator, Mr. Castro made significant improvements to the education and healthcare of his island nation."
Irish President Michael Higgins, tweeted, "Fidel Castro will be remembered as a giant among global leaders."
Alexis Tsipras, the Greek Prime minister, tweeted, "Fidel Castro sealed the world history with the struggles for freedom and independence of the people of Cuba...Laid the foundations for regional cooperation in Latin America. Deepest sympathy to the Government and the people of the Bucket."
Boris Johnson, the British foreign minister, tweeted, "Fidel #Castro's death marks the end of an era for #Cuba & the start of a new one for Cuba's people."
UK opposition leader Jeremy Corbyn said, "Fidel Castro was a massive figure in the history of the whole planet, ever since the revolution in 1959...He managed to bring good quality health services to all the people of Cuba, good quality education to all the people of Cuba and, of course, he had a foreign policy which was global, but particularly important in Southern Africa in supporting Angola against the apartheid regime...History will show that Fidel was somebody who stood up for something very, very different in the Caribbean and many independent people would say how good health care and education are in Cuba compared to many other places in the world."
José Ramón Cabañas, the Cuban ambassador to the United States, tweeted, "Hasta Siempre Comandante" -- the name of a famous Cuban song with communist themes. In English the name translates as, "until forever, commander."
Juan Orlando Hernández, the president of Honduras, tweeted, "Our condolences to President Raúl Castro, the Government and the Cuban people for the death of Commander Fidel Castro."
Hage Geingob, the president of Namibia tweeted, "The death of #Fidel signals the end of an era. Our comrade is no more but his revolutionary legacy will remain with Namibia forever."
Mikhail Gorbachev, the former Soviet leader, said in a statement, "Fidel stood up for and strengthened his country at the time of the toughest American blockade, at a time of colossal pressure on him, and all the same he led his county out of that blockade onto the road of independent development."
Narendra Modi, the prime minister of India, tweeted, "I extend my deepest condolences to the Government & people of Cuba on the sad demise of Fidel Castro. May his soul rest in peace. We stand in support with the Cuban Government and people in this tragic hour. Fidel Castro was one of the most iconic personalities of the 20th century. India mourns the loss of a great friend."
Rashtrapati Bhavan, the president of India, tweeted, "Heartfelt condolences on sad demise of Cuba's revolutionary leader, former President & friend of India, Fidel Castro."
Imran Khan, a prominent Pakistani politician and former athlete, tweeted, "1. Today the world lost an iconic revolutionary leader Fidel Castro who liberated his nation from all vestiges of imperialism. 2. Castro reasserted the Cuban nation's dignity & self worth that withstood US aggression & became a global ldr for anti colonial struggles...3. We in Pakistan will always remember with gratitude Cuba's support on the ground in the aftermath of the 2005 earthquake."
Seyed Ali Khamenei, the Ayatollah and supreme leader of Iran, tweeted, "I extensively talked with #FidelCastro in person, it is his personality to believe and rely on people. 5/27/92." A photo was included with the tweet.
Jean-Claude Junker, the president of the E.U., released a statement calling Castro a "hero for many."
"Fidel Castro was one of the historic figures of the past century and the embodiment of the Cuban Revolution," he said. "With the death of Fidel Castro, the world has lost a man who was a hero for many. He changed the course of his country and his influence reached far beyond. Fidel Castro remains one of the revolutionary figures of the 20th century. His legacy will be judged by history. I convey my condolences to the Cuban President Raúl Castro and his family and to the people of Cuba."
ABC News' Patrick Reevell, Devin Villacis and Will Gretsky contributed to this report.