President Barack Obama is in Cuba for a historic three-day visit to the island and talks with its communist leader.
He is the first sitting US president to visit since the 1959 revolution, which heralded decades of hostility between the two countries.
Speaking at the reopened US embassy in Havana, he called the visit “historic”.
Mr Obama will meet President Raul Castro, but not retired revolutionary leader Fidel Castro, and the pair will discuss trade and political reform.
President Obama’s visit is the high point of a recent easing of ties between the US and Cuba, which included the re-establishment of diplomatic missions last year.
Unsurprisingly, his first stop was the newly re-opened embassy.
Mr Obama, who is the first sitting US president to visit Cuba in 88 years, told staff: “It is wonderful to be here.”
“Back in 1928, President [Calvin] Coolidge came on a battleship. It took him three days to get here – it only took me three hours. For the first time ever, Air Force One has landed in Cuba and this is our very first stop.”
Later on Sunday, he toured Havana’s old town with his family, huddling under umbrellas to shelter from a tropical storm before visiting the national cathedral.
Cuba is one of the most exotic places I have visited. The mix of crumbling but beautiful Spanish colonial, 1950s Americana, and 1970s Soviet utilitarian, all laced with plentiful mojito cocktails and the sound of salsa, is beguiling. It’s fabulous.
I was here twice last year. The first time was just after Presidents Obama and Castro announced their desire to open a new chapter in relations; the second when Secretary of State John Kerry flew in to re-open the US embassy on the waterfront.
Now I am back for a third visit – and the cocktail that is Havana has a new ingredient. Well, two new ingredients actually. One is the smell of fresh paint. The other is shiny black tarmac on road surfaces where the potholes have been filled in.
For security reasons, no one is saying precisely where the president will go when he’s here, but here’s my top tip as a highly trained investigative reporter. The roads where the potholes have been filled in, that’s where the president will be.
Security was tight and the historic city centre looked uncharacteristically empty. One Cuban shouted: “Down with the embargo!”
President Obama responded by waving.
The 54-year-old US trade embargo is one of the main sticking points in US-Cuban relations.
It can only be lifted by the US Congress, which is controlled by Republicans who have expressed their opposition to its removal.
The other area of dispute is human rights.
The White House has insisted the president will meet political dissidents, whether the Cuban authorities like it or not.
Only hours before Mr Obama touched down, dozens of members of the dissident group Ladies in White were arrested during their weekly protest in Havana.
The group campaigns for the release of political prisoners. Mr Obama is likely to meet some of its members on Tuesday.
Despite disagreements, the visit marks a huge turnaround in US-Cuban relations.
Mr Obama and Raul Castro will sit together at a state dinner, there will be a joint news conference and they will discuss trade.
Since the two presidents announced a thaw in relation in December 2014, they have reached commercial deals on telecoms and a scheduled airline service, increased co-operation on law enforcement and environmental protection.
On Wednesday Mr Obama sent a letter on the first direct mail flight from the US to Cuba since the revolution.
Colombian newspaper Tiempo said Mr Obama put the “final nail on the coffin of the last legacy of the Cold War”, but Cuban analyst Carlos Alberto Montaner said the president “miscalculated the wasps’ nest he was getting into”.
Peruvian magazine Correo said Mr Obama must work to “make sure his progress on Cuba cannot be reversed”.
Mexican daily El Universal said “the ‘comandante’ [Fidel Castro] will go down in history as the person who fought the US to defend the revolution. The General [Raul Castro] will go down as the one who made peace”.
However, analysts say there are conflicting sentiments within Cuba’s Communist Party over hosting Mr Obama.