Thousands of protesters took to the streets in cities across Venezuela on Wednesday calling on President Nicolas Maduro to step down.

The two-hour walkout came at the urging of opposition leader Juan Guaido, who swore himself in as interim president a week ago contending that Maduro's re-election was a sham.

Guaido, the president of the National Assembly, has been recognized as president by Canada, the U.S. and several big Latin American countries.

But Maduro, 56, who took office for his second term this month, accuses Guaido of staging a U.S.-directed coup against him.

Venezuela's Supreme Court, which is stacked with Maduro allies, has barred Guaido from leaving the country after chief prosecutor Tarek William Saab announced that he was opening a criminal investigation against him.

Maduro still has the support of senior military officers and is refusing to step down. But in a New York Times op-ed published Wednesday, Guaido said he has had clandestine meetings with members of the country's military and security forces.

"The transition will require support from key military contingents. We have had clandestine meetings with members of the armed forces and the security forces," Guaido wrote. "The military's withdrawal of support from Mr. Maduro is crucial to enabling a change in government."

Embattled Venezuelan President Nicolas Maduro attends a military exercise on Tuesday as he tries to maintain support from the country's troops. (Handout photo/Miraflores Palace/Reuters)

Protesters on Wednesday held signs reading "Humanitarian aid now!" and "Freedom," and showed support for Guaido's efforts to offer amnesty to military officials who abandon Maduro.

Guaido has also called for a mass march at the weekend.

In an interview with Russia's state-owned RIA Novosti news agency on Wednesday, Maduro said he was willing to participate in talks with the opposition "for the sake of Venezuela's peace and its future," and suggested the talks could be held with mediation of other countries. Russia is one of his staunchest supporters and has offered to mediate.

Maduro also said Trump ordered "the government of Colombia and the Colombian Mafia to kill me," reprising an accusation of assassination plots that he has often made over the years. Bogota and Washington have routinely denied that.

Venezuelan opposition leader and self-proclaimed interim president Juan Guaido talks to the media in Caracas on Tuesday. (Carlos Garcia Rawlins/Reuters)

However, speculation about military action against him was fuelled this week when Trump national security adviser John Bolton carried a notepad with the words "5,000 troops to Colombia." U.S. Major General Mark Stammer, the commander of U.S. Army South, was in Colombia on Wednesday, U.S. embassy officials said.

The U.S. has emerged as Guaido's most powerful ally.

In a tweet Wednesday, Trump repeated a travel advisory from the State Department, saying Guaido was being targeted by the Venezuelan Supreme Court and telling U.S. citizens not go to Venezuela.

Maduro willing to negotiate with opposition in Venezuela following U.S. sanctions and the cutting off of oil revenues. Guaido is being targeted by Venezuelan Supreme Court. Massive protest expected today. Americans should not travel to Venezuela until further notice. —@realDonaldTrump

Later, Bolton warned traders against dealing in gold, oil or other commodities "being stolen" from the Venezuelan people, even as opponents of Maduro's government worried that a Russian plane in Caracas was preparing to ship gold out of the country.

My advice to bankers, brokers, traders, facilitators, and other businesses: don’t deal in gold, oil, or other Venezuelan commodities being stolen from the Venezuelan people by the Maduro mafia. We stand ready to continue to take action. —@AmbJohnBolton

The unusual arrival in Caracas of a Boeing 777 plane from Moscow on Monday led to speculation Maduro's government was preparing to ship more gold reserves out of the country, following shipments last year of $900 million US of gold to Turkey last year. Those shipments were part of a strategy to increase the Central Bank's liquidity.

Venezuelan lawmaker Jose Guerra, a former Central Bank economist, told the National Assembly his understanding was that the plane would take some gold reserves to Russia when it leaves. The Central Bank did not respond to a request for comment.

Sanctions 'criminal'

On Tuesday the White House announced it was giving Guaido control of Venezuela's U.S. bank accounts.

U.S. Secretary of State Mike Pompeo certified that Guaido has the authority to take control of any Venezuelan government accounts at the Federal Reserve Bank of New York or any other U.S.-insured banks. He said the certification would "help Venezuela's legitimate government safeguard those assets for the benefit of the Venezuelan people."

On Monday, the U.S. imposed sanctions on Venezuela's state-owned oil company, Petroleos de Venezuela SA, that could potentially deprive the Maduro government of $11 billion in export revenues over the next year.

Venezuela's economy is already ravaged by hyperinflation and widespread food and medical shortages that have driven millions of people to leave the country.

Maduro called the sanctions "criminal" and vowed to challenge the U.S. in court. "With these measures, they intend to rob us," he said.

Supporters of Guaido protest against Maduro Wednesday, holding signs that say 'No dictatorship.' (Carlos Barria/Reuters)

Violent street demonstrations erupted last week after Guaido declared during a huge opposition rally in Caracas that he had assumed presidential powers under the constitution and planned to hold fresh elections to end Maduro's "dictatorship."

Under Venezuela's constitution, the head of the national assembly is empowered to take on the duties of the chief executive under a range of circumstances in which the presidency is vacated. The opposition argues Maduro's re-election last May was a sham.

The previously little-known Guaido has re-invigorated the opposition movement by pushing for three immediate goals: to end Maduro's "usurpation" of power, establish a transitional government and hold a new presidential election.

The United Nations human rights office said security forces in Venezuela detained nearly 700 people in just one day of anti-government protests last week — the highest such tally in a single day in the country in at least 20 years. It said more than 40 people are believed to have been killed.

Maduro's allies blame the opposition for the violence and deny the high death toll as well as reports that minors were among those arrested.