Cuba has labelled the US a "morally decadent empire" as diplomats from more than 100 developing countries meet at the Non-Aligned Movement summit.
It is not clear whether Fidel Castro will attend a summit dinner
Cuban vice-president Carlos Lage told delegates in Havana that America wanted to impose "a worldwide dictatorship".
Foreign ministers and diplomats are working on a declaration, before heads of state and government meet on Friday.
The declaration is expected to attack US foreign policy, and to condemn the Israeli bombardment of Lebanon.
The Non-Aligned Movement is made up of 116 developing countries and aims to represent the political, economic and cultural interests of the developing world.
Doubts over Castro dinner
In a heated speech, Mr Lage slammed the United States as a "morally decadent empire".
"The ideas of limited sovereignty, humanitarian intervention, preventive war and regime change are fascist; they are not modern theories to defend freedoms and fight terrorism," he argued.
"At a time of wars and threats of more wars, the world is more and more unjust and unequal."
Late on Tuesday, Raul Castro - temporarily replacing his convalescing brother Fidel as Cuba's president - made his first appearance at the summit.
In an interview he insisted that his sibling was still giving orders, despite recent surgery for intestinal problems.
But it was unclear whether Fidel Castro would attend the six-day summit, which ends on Saturday.
One official timetable says Mr Castro will host a dinner on Friday.
But Foreign Minister Felipe Perez Roque has since said he could not be certain that the 80-year-old leader would attend.