Venezuelans began voting Sunday in a referendum that the government hopes will strengthen its century-old claim to the oil-rich Essequibo territory controlled by neighboring Guyana.

"Essequibo is ours!" say posters plastered on walls lining the streets in Caracas, part of an intensive campaign by the government of President Nicolas Maduro, who is seeking re-election next year.

"We are convinced that Essequibo is ours. It has always been ours," said Mariela Camero, 68, who voted in a working-class area of Caracas.

Voting started at 6:00 am (1000 GMT) and was to end at 6:00 pm (2200 GMT), with results expected in the early hours of Monday morning.

The Maduro government has said it is not seeking justification to invade or annex the huge territory, as some in Guyana, a former British colony, fear.

Regardless of the outcome of the vote by around 20 million eligible Venezuelans, little will change in the short term: The people of Essequibo are not voting, and the referendum is nonbinding.

But tensions have been rising since Guyana took bids in September for several offshore oil exploration blocks, and after a major new find was announced in October. Its petroleum reserves are similar to those of Kuwait, with the highest reserves per capita in the world.

Meanwhile, Maduro's government has sharpened its rhetoric and conducted military exercises in the area. 

Guyana's President Irfaan Ali said Sunday that the government was working to protect the country's borders and keep people safe. 

"I want to assure Guyanese that there is nothing to fear over the next number of hours, days, months ahead," Ali said in an address carried on Facebook, as people formed human chains dubbed "circles of union" to show their attachment to Essequibo. 

"Our first line of defense is diplomacy, and we are in a very, very strong position in this first line of defense," the president said. 

Venezuela has claimed the huge territory of Essequibo for decades — even though its 160,000 square kilometers (62,000 square miles) represent more than two-thirds of Guyana, and its population of 125,000 is one-fifth of Guyana's total.

Caracas contends that the Essequibo River to the east of the region is the natural border between the two countries, as declared in 1777 under Spanish rule, and that Britain wrongly appropriated Venezuelan lands in the 19th century.

Guyana, however, asserts that the border was set in the British colonial era and was confirmed in 1899 by a court of arbitration. It says the International Court of Justice, the UN's top judicial body, has validated this finding.

Guyana has asked the ICJ to block the referendum, saying it amounts to a violation of international rights, but in vain.

On Friday, the ICJ called on Caracas to take no action that would modify the disputed lands — but it did not mention the referendum. And Caracas had vowed to go ahead regardless.

Five questions

The referendum Sunday covers five questions, including proposals for the creation of a Venezuelan province to be called "Guyana Essequibo," giving the inhabitants Venezuelan citizenship, as well as a call to reject the ICJ's jurisdiction.

The Maduro government expects an overwhelmingly positive result, strengthening its claim.

Opposition politicians, most of whom also support the claim, have generally been reticent about the referendum.

But Maria Corina Machado, who hopes to oppose Maduro in next year's presidential elections, has called the referendum a "distraction," saying that it should be suspended and that sovereignty is not something a government should ask about — "you just exercise it."

Maduro has mobilized his party and senior members of his government in a vigorous campaign for a resounding "yes" vote.