Venezuelan President Nicolás Maduro has announced he is closing the border with Brazil on Thursday night as a row over foreign humanitarian aid continues.
The embattled leftist leader went on TV to say that 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 bus convoy to the Colombian border from the capital Caracas.
Mr Guaidó, who declared himself interim leader during anti-government protests last month and is recognised by dozens of foreign states, set off from the city on Thursday to cheering crowds.
He and his allies hope to collect food and medicine in defiance of President Maduro.
The Venezuelan military has so far managed to block shipments of US aid from coming across the border with Colombia.
Despite denying there is any humanitarian crisis, Mr Maduro announced this week that 300 tonnes of aid would be shipped to Venezuela from its ally Russia.
More than three million Venezuelans have fled in recent years as the country grapples with hyperinflation and shortages of essentials like food and medicine, the UN says.
Mr Maduro, who has been in power since 2013, has been criticised at home and abroad for his handling of the economy.
The right-wing government of President Jair Bolsonaro is among those that recognise Mr Guaidó as Venezuela’s legitimate leader, pending elections.
Presidential spokesman Gen Otávio Régo Barros said on Tuesday that, in co-ordination with the US, food and medicine would be available in the border town of Pacaraima to be collected by “the government of acting President Juan Guaidó in Venezuelan trucks driven by Venezuelans”.
“Brazil is taking part in this important international initiative to support the Guaidó government and the Venezuelan people,” he said.
Mr Guaidó has said 600,000 volunteers have already signed up to help carry aid into the country on Saturday.