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, Mr Maduro's government will hold its own event, reportedly just 300 metres away.
Mr 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.