CARACAS, Venezuela — Momentum is growing for Venezuela’s opposition movement led by lawmaker Juan Guaido, who has called supporters back into the streets for nationwide protests Saturday, escalating pressure on embattled President Nicolas Maduro to step down.

A defiant Maduro-led socialist government has called on its own loyalists to flood the streets waving flags to celebrate the 20th anniversary of the Bolivarian revolution launched by the late Hugo Chavez.

The dueling demonstrations will play out amid a political standoff in its second week of heightened tensions — and with the potential to spark violent clashes between the opposition and security forces.

Guaido has turned down offers from the presidents of Mexico and Uruguay to negotiate with Maduro. In a letter Guaido urged both presidents to back Venezuela’s struggle.

“At this historical moment that our country is going through, to be neutral is to be on the side of the regime that has condemned hundreds of thousands of human beings to misery, hunger and exile — including death,” he said.

Guaido declared himself interim president last week before tens of thousands of cheering supporters and vowed to end Maduro’s “dictatorship.” His claim to the presidency is backed by the United States and some two dozen other nations.

The opposition seeks to usher in a transition by holding democratic elections, Guaido said in the letter to Uruguayan President Tabare Vazquez and Mexico’s President Andres Manuel Lopez Obrador.

The United States has also rejected the offers from Mexico, Uruguay and the Vatican to mediate a dialogue.

Vice President Mike Pence on Friday met with exiled Venezuelans in Miami, reassuring them the Trump administration would continue to weaken Maduro.

President Donald Trump’s national security adviser John Bolton tweeted Thursday that Maduro and his top advisers should retire to “a nice beach somewhere far away from Venezuela.” Bolton’s talk turned tougher Friday in an interview with conservative radio talk show host Hugh Hewitt in which he warned that it could be a beach area more like Guantanamo.

Maduro remains dug in, blaming the White House for openly backing what he calls a coup to remove him from power and exploit his country’s vast oil wealth. He retains support from powerful allies, including Russia and China, but is growing increasingly isolated.

Maduro on Friday continued a show of might as commander-in-chief.

“We’re facing the greatest political, diplomatic and economic aggression that Venezuela has confronted in 200 years,” Maduro told several hundred troops standing in formation around armored vehicles.