Speaking Sunday at the closing of the party's first National Conference, Castro said abandoning the island nation's one-party system would be "to legalize the party or parties of the [U.S.] empire," El Nuevo Herald reported.
At the two-day, closed-door gathering, the more than 800 delegates approved replacement of as much as 20 percent of 115 Central Committee members in the next five years. That could highlight younger leaders who could succeed Castro, 80.
Castro urged party members to become more "democratic" in debating Cuba's problems and called for more openness in the party and the media, but clearly within limits.
Cuba's mass media, all party- or state-controlled, need to report "with responsibility and the most strict veracity," he said, "not in the bourgeois style, full of sensationalism and lies, but with proven objectivity and without useless secretiveness."
He also cited the need for hard work, ethics and discipline and said party officials shouldn't interfere with decisions that should be left to the government.
"The only thing that can defeat the revolution and socialism in Cuba would be our incapacity to correct the errors committed in the last 50 years … and those that we could make in the future," he said.
Castro also criticized corruption -- "one of the principal enemies of the revolution, much more prejudicial than the subversive and meddlesome programs of the U.S. government," he said, referring to Washington's pro-democracy programs in Cuba.
"The party will definitively assume the conduct" of the fight against corruption, he said, without elaborating.