In a letter to the Vatican (search) published Sunday on the front page of Juventud Rebelde newspaper, Castro called the pope's passing "sad news" and expressed "the most heartfelt condolences of the Cuban people and government."
"Humanity will preserve an emotional memory of the tireless work of His Holiness John Paul II in favor of peace, justice and solidarity among all people," Castro wrote.
The Cuban leader also highlighted the pope's historic January 1998 visit to Cuba, saying it will remain "engraved in the memory of our nation as a transcendent moment in relations between the Vatican State and the Republic of Cuba."
Cuba became officially atheist in the years after the 1959 revolution that brought Castro to power, but the government removed references to atheism in the constitution more than a decade ago and allowed religious believers to join the Communist Party.
Cuban Catholics gathered Sunday in Havana's towering cathedral for Mass, led by Cardinal Jaime Ortega, the island's top Roman Catholic churchman.
Ortega, who has been mentioned as a possible successor to the pope, extolled John Paul's virtues and his message of peace, love and justice, saying, "The pope stirred humanity, just like Jesus did."
He later spoke with reporters but declined to speculate on likely successors to the pope, saying the decision will not be based on "preferences for a certain person, country or continent."
"The selection of the pope isn't produced from a candidacy," Ortega said. "The church ... looks for what the church needs at this moment, and that reflection will be decisive."
John Paul's death "leaves an enormous void ... but the church continues," he added.
Ortega plans to celebrate a funeral Mass for the pope at the Havana cathedral Monday evening then travel to Rome later that night to attend the pope's funeral there and participate in the conclave of cardinals that will elect John Paul's successor.
The Cuban government said it would send a high-level delegation to the pope's funeral, but it was not yet clear who would head it.
State-run media covered little of the news leading up to the pope's death, but once his death was announced, the news was broadcast across the island on the government's Radio Rebelde.
Much of Sunday's Juventud Rebelde was dedicated to the pope, with the state decree about official mourning on the island printed on the front page.
The Cuban flag was to hang at half-mast on public buildings and military installations for three days, and several events, including anniversary celebrations for communist organizations and baseball games, were suspended.
The decree praised the pope for all of his efforts "in favor of solutions for many social ills affecting humanity," as well as for publicly criticizing four decades of U.S. trade sanctions against Cuba.
Ortega said he thought the government was "very warm" in its reaction to the pope's death. And though he expected the declaration of official mourning, he said he was very surprised to hear that baseball games — a national passion for Cubans — had been suspended.
"It's a sign of special solidarity in this moment," he said. "It's a beautiful sign."
Later in the day, more than 200 Cubans lined up outside the Vatican's diplomatic offices in Havana to sign a book of condolences for John Paul.
Archbishop Luigi Bonazzi, the Vatican's ambassador to Cuba, greeted those in line, including nuns, regular citizens and wives of Cuban political prisoners.
"One can see that the friendship the pope showed to Cuba on numerous occasions endures," he told reporters.
Cuban officials — including Foreign Minister Felipe Perez Roque and Caridad Diego, the director of religious affairs for the island's Communist Party — also stopped by to sign the book. Bonazzi personally thanked Perez Roque for the government's gestures of solidarity.