Cuban President Raul Castro has defended the one-party system in a speech at the conference of the ruling Communist Party.
Mr Castro said allowing other political parties would threaten Cuba's independence and the socialist system established by the 1959 revolution.
But he said it was necessary to promote more democracy and open debate within the Communist Party.
He also reaffirmed plans to limit political terms to 10 years.
Mr Castro, 80, has launched widespread economic reform since taking over from his brother Fidel in 2008.
But in his address, he ruled out any change to the one-party system, criticising "those who hoped the conference would mark the start of the dismantling of the social and political conquests of the revolution".
"Giving up the principle of one party would simply amount to allowing the party or parties of imperialism on national soil," he said.
Mr Castro added that those who thought Cuba should return to multi party democracy ignored "the history of permanent aggression, economic blockade, interference and media siege" that Cuba had faced from the US.
However, he warned that corruption and a failure to eradicate "the errors of the past" represented the most serious threat to the future of the socialist system.
And he said the Communist Party should promote greater democracy and open debate within its own ranks and in the mass media.
President Castro also repeated his plan to limit leaders to two five-year terms, saying the measure would be implemented gradually without waiting for a reform to the constitution.
The proposal - first announced last year - represents a major change in Cuba where senior positions are dominated by veterans of the 1959 revolution, known as the "historic generation".
Fidel Castro ruled for nearly half a century before handing power to his brother because of ill health.
Raul Castro did not say if his own term as president would be limited.