President Barack Obama arrived to small but cheering crowds at the start of a historic visit to Cuba that opened a new chapter in US engagement with the island's Communist government, after decades of hostility between the former Cold War foes.
The three-day trip -- the first by a US president to Cuba in 88 years -- is the culmination of a diplomatic opening announced by Obama and Cuban President Raul Castro in December 2014.
The visit ends an estrangement that began when the Cuban revolution ousted a pro-American government in 1959.
"It's a historic opportunity to engage directly with the Cuban people," Obama told staff at the newly reopened US Embassy.
Obama's first words to the Cuban people came in a message on Twitter, a social media service few Cubans can use regularly because of government restrictions on internet access.
"Qu bol Cuba?" he said, using Cuban slang for "What's up?"
"Just touched down here, looking forward to meeting and hearing directly from the Cuban people."
Groups of Cubans watched the motorcade from balconies and backyards as Obama was driven downtown, where a small crowd of Cubans braved a tropical downpour and tight security.
They chanted "Viva Obama, Viva Fidel" as the president and his family left after eating dinner in a rundown neighbourhood.
Obama, who abandoned a long-time US policy of trying to isolate Cuba, wants to make his policy shift irreversible even if a Republican wins the White House in November.
However major obstacles remain to full normalisation of ties, and the Democratic president's critics say the visit is premature.
On Sunday, one bystander shouted: "Down with the blockade", in reference to the US embargo in place for 54 years that remains the top irritant for Cubans.
Obama has asked Congress to rescind the embargo but has been blocked by the Republican leadership.
Underscoring the ideological divide that persists between Washington and Havana, Cuban police, backed by hundreds of pro-government demonstrators, broke up the regular march of a leading dissident group, the Ladies in White, and detained about 50 people just hours before Obama arrived.
Obama arrived at Havana's Jose Marti International Airport and was met by Cuban Foreign Minister Bruno Rodriguez.
The formal welcoming ceremony will be at the presidential palace.
US officials appeared unfazed by Castro's absence from the airport welcome, even though he personally met and greeted Pope Francis in September.
Republican presidential front-runner Donald Trump tweeted that Obama's visit was a "big deal" but he got "no respect".
Obama will hold talks with Castro -- but not his brother Fidel, the revolutionary leader -- and speak to entrepreneurs on Monday.
He meets privately with dissidents, addresses Cubans live on state-run media and attends an exhibition baseball game on Tuesday.
The trip makes Obama the first sitting American president to visit Cuba since Calvin Coolidge arrived on a battleship in 1928.