Obama and Castro, separated by the presidents of Ecuador and El Salvador, stood in the second of three rows at a Panama City convention center yesterday as the country's national anthem was sung.
The sight of Obama and Castro in the same room instantly became a potent symbol of their bid to renew diplomatic ties that were severed in 1961. It was the first time that a Cuban leader attended the summit in its 21-year history.
The 30-odd leaders were then heading to a seaside dinner in a complex of ruins dating from the era of the Spanish conquistadors.
Obama and Castro met once before, when they already shook hands at Nelson Mandela's funeral in December 2013. But US officials said the widely-anticipated discussion would take place today.
The last US and Cuban leaders to meet were Dwight Einsenhower and dictator Fulencio Batista in 1956, three years before Fidel Castro's revolution.
"We certainly do anticipate that they will have the opportunity to see each other at the summit tomorrow, to have a discussion," senior Obama advisor Ben Rhodes told reporters.
Rhodes said Obama and Castro had already discussed the ongoing negotiations and the upcoming summit by telephone Wednesday -- their second phone call since December.
In a move that could irritate Havana, Obama held a closed-door discussion before the summit with dissident lawyer Laritza Diversent and political activist Manuel Cuesta Moura, along with a dozen other activists from the Americas.
While declaring that the days of US meddling in the region were over, the first African American president of the United States launched a defiant and markedly personal defense of Washington's support for opposition groups.
"As you work for change, the United States will stand up alongside you every step of the way."
Turning to Cuba, Obama said that even as "a new chapter" in relations was launched, "we'll have our differences, government to government, with Cuba on many issues.
