HAVANA (Reuters) - Cubans flocked to the polls on Sunday in a vote expected to approve a new constitution that institutes modest economic and social changes while maintaining the one-party socialist system.

Under a bright sun and the watchful gaze of uniformed grammar school children guarding ballot boxes, 81 percent of the 8.7 million electorate had cast votes by 5 p.m., an hour before polls closed, according to the national electoral commission.

Results are due to be announced on Monday.

No incidents were reported at polling stations, though there were unconfirmed social media accounts of police blocking small groups of dissidents from protesting in eastern Cuba and Havana.

Debate over the constitution has dominated the country’s politics for months, even as it struggles with economic stagnation and as the deepening crisis in Venezuela brings its ally into the crosshairs of U.S. President Donald Trump’s administration.

White House national security adviser John Bolton ripped the referendum on Twitter as “another ploy of the Cuban regime to cover up its repression and tyranny.”

Cuban President Miguel Diaz-Canel said the vote was taking place as events in Venezuela showed the “imperialist threat” facing the region.

“Today we are going to win ... It will also be an important victory for Venezuela,” he said after waiting in line in Havana to cast his ballot.

“I think the Americans in their arrogance have been wrong. People are waking up when they see these nefarious practices and realize what is behind this show,” he said.

Slideshow ( 5 images )

Former President Raul Castro, who remains head of the Communist Party, kept an unusually low profile in apparent deference to his successor Diaz-Canel.

State media screened footage of Castro casting his vote and chatting with school children at a polling station, but he made no comments to reporters.

ONE-SIDED CAMPAIGN

Cuban evangelicals were expected to vote against the proposed constitution because they fear it opens the way for gay marriage while the Roman Catholic Church has also voiced criticism.

A campaign to reject the constitution because it entrenches Communism was launched on social media by dissidents and exiles.

“This 24th the Cuban people’s rejection of the system imposed by the Communist party will be felt,” Rosa Maria Paya, leader of the Cuba Decide organization of government opponents inside and outside Cuba, wrote on Twitter.

The government orchestrated a grassroots debate on a draft of the new constitution last year, but since it approved a final version for Sunday’s vote, it has used its monopoly of traditional media and public spaces to press for approval and censor other points of view.

Foreign and local observers said they expect between 70 percent and 80 percent of voters to ratify the new constitution. They expect more no votes and abstentions than in 1976, when the current version of the constitution was approved by 97.8 percent of the electorate.

Slideshow ( 5 images )

“Cubans had the opportunity to debate the constitution in their neighborhoods and workplaces,” Lazaro Rodriguez, 58, said as he shopped in a Havana produce market.

“It updates our economic system, which is not very good, but we are trying to modernize and that is very positive,” he said, adding that he would vote yes.

The proposed changes reflect the gradual opening of Cuba since the fall of its former benefactor, the Soviet Union.

There are references to markets and recognition of private property, foreign investment, small businesses, gender identity, the internet, the right to legal representation upon arrest and habeas corpus.

The new constitution would also restructure government, adding a prime minister and setting term limits for the president, among other changes.