HAVANA — Raul Castro turned over Cuba’s presidency Thursday to a 57-year-old successor he said would hold power until 2031, a plan that would place the state the Castro brothers founded and ruled for 60 years in the hands of a Communist Party official little known to most on the island.

Castro’s 90-minute valedictory speech offered his first clear vision for the nation’s future power structure under new President Miguel Mario Diaz-Canel Bermudez. Castro said he foresees the white-haired electronics engineer serving two five-year terms as leader of the Cuban government, and taking the helm of the Communist Party, the country’s ultimate authority, when Castro leaves the powerful position in 2021.

“From that point on, I will be just another soldier defending this revolution,” Castro said. The 86-year-old general broke frequently from his prepared remarks to joke and banter with officials on the dais in the National Assembly, saying he looked forward to having more time to travel the country.

In his own half-hour speech to the nation, Diaz-Canel pledged to preserve Cuba’s communist system while gradually reforming the economy and making the government more responsive to the people.