* Visit was unthinkable until Obama-Castro agreement
* Tight security and welcome signs await U.S. president
* No visit with Fidel Castro planned (Recasts with Obama arriving)
By Matt Spetalnick, Daniel Trotta, Jeff Mason and Frank Jack Daniel
HAVANA, March 20 (Reuters) - U.S. President Barack Obama arrived in Cuba on Sunday on a historic visit, opening a new chapter in U.S. engagement with the island’s Communist government after decades of animosity between the former Cold War foes.
Obama landed at Havana’s Jose Marti International Airport aboard Air Force One, the presidential jet with “United States of America” emblazoned across its fuselage, a sight almost unimaginable before the detente of December 2014.
The three-day trip, the first by a U.S. president to Cuba in 88 years, is the culmination of a diplomatic opening announced by Obama and Cuban President Raul Castro in December 2014, ending a Cold War-era estrangement that began when the Cuban revolution ousted a pro-American government in 1959.
Obama, who abandoned a longtime U.S. policy of trying to isolate Cuba, wants to make his shift irreversible. But major obstacles remain to full normalization of ties, and Obama’s critics at home say the visit is premature.
Traveling with first lady Michelle Obama, her mother and their daughters, Sasha and Malia, the president will mostly play tourist on his first night on the Caribbean island, taking in the famous sights of Old Havana.
He will hold talks with Raul 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 carries both symbolism and substance after decades of hostility between Washington and Havana.
It makes Obama the first sitting American president to visit Cuba since Calvin Coolidge arrived on a battleship in 1928.
It is also another major step in chipping away at remaining barriers to U.S.-Cuba trade and travel and developing more normal relations between Washington and Havana.
Since rapprochement, the two sides have restored diplomatic ties and signed commercial deals on telecommunications and scheduled airline service.
Major differences remain, notably the 54-year-old economic embargo of Cuba. Obama has asked Congress to rescind it, but the move 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, detaining about 50 people just hours before Obama was due to arrive.
Additional reporting by Jeff Mason aboard Air Force One, and Marc Frank and Nelson Acosta in Havana; Editing by Mary Milliken, Alan Crosby and Peter Cooney