One of the defining leaders of the 20th century, Fidel Castro was seen as both pariah and people's champion.
Prominent figures have paid tribute to the former Cuban president, who has died aged 90.
Foreign Secretary Boris Johnson said Mr Castro was a "historic if controversial figure" and his death marked "the end of an era for Cuba and the start of a new one for Cuba's people".
He said: "The UK expresses its condolences to the government and people of Cuba, and to the former president's family. Fidel Castro's death marks the end of an era for Cuba and the start of a new one for Cuba's people.
"Fidel Castro's leadership of the 1959 Cuban Revolution marked him out as an historic if controversial figure.
"The UK will continue to work with the government of Cuba on a wide range of foreign policy priorities, including on human rights."
Prominent figures from the left of British politics praised Mr Castro.
Labour leader Jeremy Corbyn praised his "heroism" and told the Press Association: "Fidel Castro was a massive figure in the history of the whole planet."
Former London mayor Ken Livingstone said the former Cuban leader was an "absolute giant of the 20th century", and blamed the US for the restrictions on civil liberties under his leadership.
He told Radio 4's Today programme: "I'm sure they will, over time, move towards something like a traditional west European democracy. It could have happened a lot earlier if you hadn't had, the entire time, a blockade by America, attempts to overthrow the regime, eight assassination attempts authorised by American presidents."
Mr Livingstone said Cuba could reform now it was not under threat of American invasion "even if Trump goes a little bit bonkers".
He admitted "of course Fidel did things that were wrong", adding: "Initially he wasn't very good on lesbian and gay rights, but the key things that mattered was that people had a good education, good healthcare and wealth was evenly distributed.
"He was not living as a billionaire laundering money off into a Panamanian bank account or anything like that, he was good for the people."
Former Labour MP George Galloway tweeted a picture of himself with Castro, writing: "You were the greatest man I ever met Comandante Fidel. You were the man of the century. Hasta la Victoria Siepmre. Orden. RIP."
He later added: "Who knew so many sickos were out there waiting for Fidel's death? Dogs can dance on the lion's grave. But they can never be a lion."
Former Labour Cabinet minister and anti-apartheid leader Peter Hain, now Lord Hain, said: "Although responsible for indefensible human rights and free speech abuses, Castro created a society of unparalleled access to free health, education and equal opportunity despite an economically throttling USA siege.
"His troops inflicted the first defeat on South Africa's troops in Angola in 1988, a vital turning point in the struggle against apartheid."
Irish president Michael D Higgins was among a host of world leaders who also paid tribute, saying Castro guided Cuba "through a remarkable process of social and political change, advocating a development path that was unique and determinedly independent".
He added: "Fidel Castro will be remembered as a giant among global leaders whose view was not only one of freedom for his people but for all of the oppressed and excluded peoples on the planet."
Indian Prime Minister Narendra Modi said: "Fidel Castro was one of the most iconic personalities of the 20th century. India mourns the loss of a great friend. I extend my deepest condolences to the Government and people of Cuba on the sad demise of Fidel Castro. May his soul rest in peace."
Mexican President Enrique Pena Nieto said: "Fidel Castro was a friend of Mexico, promoter of a bilateral relationship based on respect, dialogue and solidarity."
Salvador Sanchez Ceren, President of El Salvador, tweeted: "With deep sorrow we received news of the death of my dear friend and eternal companion, Commander Fidel Castro Ruz."
Foreign Secretary Boris Johnson said: "Fidel Castro's death marks the end of an era for Cuba and the start of a new one for Cuba's people."