HAVANA (Reuters) - Cubans vote on Sunday to ratify two official lists of candidates for the national and provincial assemblies, the final step in a process that is set to culminate with Raul Castro stepping down as president on April 19.

The following is an outline of Cuba’s one-party socialist system, one of the last in the world:

* Cuba is a one-party socialist republic, in which political power is vested solely in the Cuban Communist Party (PCC). The system is enshrined in the Cuban Constitution approved by referendum in 1976. Another referendum in 2002 made socialism “irrevocable.”

* The PCC was founded in 1965 by merging various parties and revolutionary groups under Fidel Castro’s leadership. All other parties were banned. Dissidents are often persecuted through a Penal Code that sanctions offences like “enemy propaganda,” “illicit association” and “clandestine printing.”

* The constitution designates the PCC as the “vanguard of the Cuban nation ... which organises and directs common efforts toward the higher goals of construction of socialism and the advance toward communist society.”

The PCC says it is influenced by the teachings of Cuba’s 19th century independence hero Jose Marti and by Marxist-Leninist ideology. At the last official count in April 2016, it had around 800,000 members.

* Raul Castro, like his late brother Fidel, holds the three top political posts on the island: head of state (as president of the Council of State), head of government (as president of Council of Ministers) and first secretary of the party.

He is due to step down as head of state and government in April but is expected to retain the party post, possibly until the next PCC congress in 2021.

* The PCC holds a policy-making congress every five years to set political and economic strategies. The congress also elects a Central Committee which in turn elects a Politburo, Cuba’s most powerful political leadership body.

* The National Assembly is the Cuban legislature, with around 600 delegates selected every five years by party-controlled candidacy commissions and ratified by ordinary Cubans in a ballot.

Half of the delegates emerge from municipal assemblies, intended to ensure the party keeps in touch with grassroots issues. The remainder are officials and personalities from the arts, sport and other sectors. Most delegates are PCC members.

* The National Assembly usually meets twice a year for a few days. Every five years, it approves a slate of around 30 members of the Council of State, the highest executive body headed by the president, a first vice president and five second-tier vice presidents.

* All Cuban citizens over the age of 16 years are eligible to vote. Voting is not obligatory but is presented by the island’s authorities as a “patriotic duty.” Municipal elections held every 2.5 years are the only vote in which ordinary Cubans get to choose between two or more candidates.

* Cuban society is organised into “mass organisations” of workers, students, women and farmers. The biggest is a network of neighbourhood block committees, known as the Committees for the Defense of the Revolution (CDR), tasked with mobilizing support for the government and defending the political system against crime and “counter-revolution.”

Critics say they facilitate control over the population. The CDRs have lost influence over the last decade, locals say.