URENA, Venezuela (AFP) — Venezuela’s Opposition leader Juan Guaido claimed Saturday a first shipment of humanitarian aid has reached Venezuela, defying a border blockade by President Nicolas Maduro as a stand-off over the entry of food and medical aid turned violent.

Venezuelan National Guard forces fired tear gas and rubber bullets at several crossing points on the Colombia border, pinning down demonstrators seeking to reach humanitarian aid stockpiled on the other side.

Guaido formally launched the distribution operation at a warehouse at the Tienditas border bridge, in Cucuta, Colombia.

“The humanitarian aid is definitely going to Venezuela in a peaceful and calm manner to save lives at this time,” said Guaido, joined for the launch by the presidents of Colombia, Chile and Paraguay.

He later announced that an aid truck had crossed into Venezuela on its southern border with Brazil. However, AFP reporters there witnessed a truck halted at the border which had yet to cross. People from a local indigenous community were headed to the crossing to try to pressure troops to let in the aid.

Early Saturday two large trucks carrying eight tonnes of emergency aid left Boa Vista in Brazil en route to the Venezuelan border. The vehicles are driven by Venezuelans and escorted by Brazilian police, organisers said.

“We officially announce that the first shipment of humanitarian aid has now entered our border with Brazil. This is a great achievement, Venezuela!” wrote Guaido in a tweet.

Demonstrators waiting for aid in Urena on the Venezuelan side of the Colombian border clashed with National Guard troops, who fired tear gas and rubber bullets.

In the confusion, four National Guard members abandoned their posts and crossed into Colombia.

Violence later spread to nearby San Antonio del Tachira, after soldiers blocked hundreds of people from reaching a border bridge to bring in aid. AFP reporters said shots were fired in the town after armed men arrived on motorcycles.

Guaido has vowed humanitarian aid would enter his country on Saturday despite a blockade.

His supporters plan to drive the aid from Colombia into Venezuela at the closed border crossings supported by a flood of volunteers and accompanied by Catholic priests in an attempt to avoid arrest.

Socialist leader Maduro has rejected the aid, which he’s dismissed as a show and pretext for a US invasion.

Meanwhile, thousands of opposition and pro-Maduro supporters turned out at rival rallies in the capital Caracas.

Humanitarian aid has become the centrepiece of the stand-off between Maduro and Guaido, the 35-year-old leader of Venezuela’s National Assembly who declared himself interim president exactly one month ago Saturday. Guaido has accused Maduro of rigging his re-election and is demanding a new vote.

Venezuela is gripped by a humanitarian crisis that has seen poverty soar during years of recession.

As many as 300,000 Venezuelans are in dire need of food and medicine after years of shortages and malnutrition, according to Guaido.

United Nations figures show that some 2.7 million people have fled Venezuela since 2015 amid the crisis, and some 5,000 Venezuelans emigrate from their country each day.

On the eve of the face-off Guaido defied a government ban on leaving the country and attended the “Venezuela Live Aid” concert organised by British billionaire Richard Branson just over the border in Colombia.

Guaido sensationally claimed that the Venezuelan military, whose high command has repeatedly declared absolute loyalty to Maduro, “participated in this process” to get him into Colombia.

Hours later, Caracas said it had closed much of the Colombian border, citing threats to Venezuela’s security.