Venezuela's opposition leader Juan Guaido told tens of thousands of supporters on Tuesday that desperately-needed humanitarian aid would be brought into the country on February 23, despite opposition from President Nicolas Maduro.

"It's sure that the humanitarian aid will enter Venezuela because the usurper will have no choice but to leave Venezuela," said self-proclaimed acting president Guaido.

US aid has been piling up in Colombia at the border with crisis-hit Venezuela but until now the bridge border crossing has been barricaded by the military, under Maduro's orders.

"We have almost 300,000 Venezuelans who will die if the aid doesn't enter. There are almost two million at health risk," said Guaido, recognized by 50 countries after declaring himself as Venezuela's interim leader.

Taking his authority from the constitution, National Assembly leader Guaido considers Maduro "illegitimate" following his reelection last year in a poll widely viewed as fraudulent.

Guaido is trying to force the socialist leader from power so he can set up a transitional government and hold new presidential elections.

Venezuelans have faced shortages of basic necessities such as food and medicine as the economy collapsed under Maduro. Some 2.3 million Venezuelans have fled since 2015, as hyperinflation rendered salaries and savings worthless.

Maduro denies there is a humanitarian emergency and has branded the crisis a "political show" and pretext for a US-led invasion.

Foreign Minister Sergei Lavrov warned the US "against all interference in Venezuela's domestic affairs including the use of force threatened by Washington and which is in violation of international law."

"It's not the first time Venezuela is going to be liberated from a tyrant," said Guaido.

"Here is a direct order to the armed forces: allow in the humanitarian aid once and for all (and) end the repression."

The fate of tons of aid piling up in Colombian collection centers at the border with Venezuela has become central to the power struggle between Guaido and Maduro, who is backed by the powerful armed forces.

"The military are the ones keeping Maduro in power. I call on them to reflect and take the side of the people suffering from hunger," said Maria Ballera, 75.

Guaido had called the Youth Day demonstrations across the country in part to honor 40 people killed in anti-government rallies last month.

While waiting for him to speak in eastern Caracas, his supporters chanted: "Freedom!"