Havana (AFP) - Cuban President Raul Castro rejected rapid privatization and vowed never to pursue "shock therapy," setting the tone for a Communist Party congress reviewing progress in revamping the island's moribund Soviet-style economy.

"Cuba will never permit the application of so-called shock therapies, which are frequently applied to the detriment of society's most humble classes," he said in a lengthy speech opening the congress, which takes place every five years and will stretch on for several days.

The main political event in a one-party system that brooks no dissent, the congress comes less than a month after US President Barack Obama's historic visit and with Havana normalizing ties with the United States, its longtime Cold War foe.

But if Cubans and international observers were hoping for a sign of significant political and economic change at the meeting, Castro -- the man responsible for setting in motion economic reforms -- swiftly signaled otherwise.

"The neoliberal formulas that promote accelerated privatization of state assets and social services such as education, health and social security will never be applied under Cuban socialism," warned Castro, who formally took over from his ailing brother Fidel in 2008.

Castro, 84, defended the slow pace of change to the island's economy, which has only cautiously and gradually opened up to some private entrepreneurship and foreign investment.

"Private enterprise will evolve within defined limits and will provide a complementary element of the country's economic framework," he added, in remarks broadcast to the country's 11.3 million people on state television.

Warming to the theme, he defended Cuba's one-party system that has seen dissidents arrested and locked up: "If one day they manage to divide us, it will be the beginning of the end."

And Castro again blamed Washington's more than five-decade-old embargo on the island for its economic impact on Cuba. The United States and Cuba are slowly normalizing ties, even reopening embassies in each other's capitals, but the trade embargo on Cuba remains.

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- Held in secret -

In contrast with the last party congress, which was preceded by a wide-ranging public debate, this one was being held in secret, with only the state-controlled press allowed to cover the proceedings.

The last congress, in 2011, introduced significant reforms of the island's economy, cracking open the door to small-scale private enterprise and foreign investment.

This one, the Seventh Congress, had raised expectations that it could set the stage for accelerated political and economic changes following the rapprochement with Washington.

But ahead of the meeting, Cuban authorities poured cold water on those hopes, signaling that continuity would be the watchword at the four-day, closed-door session involving 1,000 delegates and another 3,500 invited participants.

For the first time, the agenda of the Congress was kept secret and will not be debated publicly, something that has surprised even the ruling Communist Party's rank and file.

The congress was expected to approve an economic and social development program for the 2016-2030 period.

Cuban diplomacy has been very active over the past five years, its efforts crowned by the spectacular rapprochement with the United States and a dialogue that is now underway with the European Union.

But Cuba's opening to the West is also proving to be a gradual one, reflecting Raul Castro's caution as the island undergoes a transition to a new generation of leaders after more than 55 years under the Castro brothers.

"The fundamental problem is that there is a lack of consensus on the country's development strategy, on the changes that are necessary and on the pace at which they should be made," said Mauricio de Miranda, a Cuban economist at Colombia's Javeriana University, ahead of the meeting.

Experts say that Cuban authorities appear more interested in making it clear that there will be no capitalistic restoration, not now and not when Cuba's revolutionary leaders leave the scene.