Venezuela's embattled President Nicolás Maduro has ordered the closure of the border with Brazil amid a row over foreign humanitarian aid.
The leftist leader went on TV to say he was also considering shutting the border with Colombia to stop the opposition bringing in relief.
He denies any crisis and calls the aid delivery plans a US-orchestrated show.
Opposition leader Juan Guaidó is leading a convoy from the capital, Caracas, to the Colombian border.
Later on Friday, a concert will be held on the Colombian side of the border to raise money for Venezuela. At the same time, the government will hold its own event, reportedly just 300 metres away.
Guaidó, head of the National Assembly, declared himself interim leader during anti-government protests last month and is recognised by dozens of countries, including the US and most Latin American nations.
He and his allies hope to collect food and medicine being gathered in neighbouring Brazil and Colombia in defiance of President Maduro.
Flanked by Venezuela's defence minister and other top military commanders, Maduro announced that the border with Brazil would be closed "completely and absolutely" until further notice on Thursday night.
The crossing usually closes at night and would normally open at 08:00 local time (12:00 GMT), reports BBC.
After the announcement, many Venezuelans rushed across the frontier to the Brazilian city of Pacaraima to stock up on supplies, Brazilian news portal G1 reports.
The right-wing Brazilian government of President Jair Bolsonaro is among those that recognise Guaidó as Venezuela's legitimate leader, pending elections.
On Tuesday, Brazil announced that, in co-ordination with the US, food and medicine would be available in Pacaraima to be collected by "the government of acting President Juan Guaidó in Venezuelan trucks driven by Venezuelans".
British entrepreneur Sir Richard Branson has organised a huge benefit concert near the Tienditas Bridge crossing, on the Colombian side of the border at Cúcuta, to raise money for Venezuela.
About 250,000 people are expected at a gig which organisers hope will raise about $100m (£77m) to buy food and medicine for Venezuelans, Reuters news agency reports.
Not to be outdone, the Venezuelan government has erected a stage on its side of the crossing.