Castro and Maduro at May Day Rally in Havana

Venezuela's President Nicolas Maduro joined President Raul Castro at a huge May Day rally in Havana on Friday in a show of socialist unity minus the usual anti-imperialist rhetoric.

Amid a tropical rain shower, Castro saluted thousands of people as they passed the reviewing stand at the Plaza of the Revolution in the Cuban capital.

Castro and Maduro, the leader of Cuba's closest ally, ignored the rain as the hour and a half long event unfolded in the heart of Havana.

The annual rally was held this year against a backdrop of a thaw in Cuban relations with the United States and a deepening crisis in oil-rich Venezuela, which has undercut public support for Maduro.

"Some steps have already been taken to re-establish diplomatic relations between Cuba and the United States, but we have a long and difficult road to travel," said Ulises Guilarte, the head of the Cuban Workers Union, the sole speaker at the rally.

"We will only advance toward the normalization of bilateral relations on a basis of respect for the sovereignty, the independence of Cuba, which includes the lifting of the blockade and the return of territory usurped by the U.S. naval base at Guantanamo," Guilarte said.

But his speech was notable for the absence of the anti-American rhetoric of past years. 

Negotiations on restoring relations have been underway since Castro and U.S. President Barack Obama's surprise announcement December 17 that they would work to normalize ties severed for more than 50 years.

In a potent symbol of the changing times, Castro attended a Summit of the Americas in Panama last month for the first time.

Cuba's opening to the United States fits with a broader effort to overhaul the island's Soviet-style economy and attract foreign investment needed to spur growth.

Since the collapse of the Soviet Union, Cuba has relied heavily on Venezuela for cut-rate oil supplies and investment, a lifeline that could be threatened by its ally's economic problems.

But Guilarte, saluting Maduro's presence at the May Day rally, warned: "We firmly reject all external meddling, as well as destabilizing actions, violence and war that violate constitutional order, sovereignty, independence and the self-determination of the Venezuelan people."

Doctors and medics who were on the frontlines of the Ebola epidemic in west Africa headed the march through Havana.

More than 2,000 members of unions and political groups from around the world also were invited to attend.