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 convoy to the Colombian border from the capital Caracas.
Mr Guaidó declared himself interim leader during anti-government protests last month and is recognised by dozens of foreign states, including the US.
Scuffles broke out and tear gas was fired when the convoy of buses and cars was briefly stopped by security forces on a road near Mariara, west of Caracas, but they later moved on.
Meanwhile, after the closure of the border with Brazil was announced, many Venezuelans rushed across the frontier to the Brazilian city of Pacaraima to stock up on supplies, Brazilian news portal G1 reported.
Queues of cars and people continued late into the evening, witnesses said.
Mr Guaidó 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.
Is support for Guaidó growing?
Venezuela’s military has so far resisted calls to abandon President Maduro and support his rival.
However, on Thursday former military intelligence chief Hugo Carvajal recognised the opposition leader as “president in charge”. In a video address posted online he issued a stinging rebuke to Mr Maduro.
“You have killed hundreds of young people in the streets for trying to claim the rights you stole – this without even counting the dead for lack of medicines and security,” he said.
Mr Carvajal, a congressman, called on the military to break with the president and to allow humanitarian aid in.
In another development, Mr Guaidó’s aides in Washington said 11 Venezuelan diplomats based in the US had defected and declared their support for him.
Flanked by Defence Minister Vladimir Padrino Lopez and other top military commanders, Mr Maduro announced that the border with Brazil would be closed “completely and absolutely” from 20:00 (23:00 GMT) until further notice.
The president also added: “I don’t want to take any decision of this type but I am evaluating it, a total closure of the border with Colombia.”
He says the delivery of aid is part of a US-led attempt to depose his government and seize Venezuela’s oil reserves.
The right-wing Brazilian 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.
Venezuela earlier closed its sea and air border with Curacao, a Dutch Caribbean island off Venezuela’s north coast which is planning to host US aid.
Where is Guaidó’s convoy going?
It is expected to travel some 800-900km (500-560 miles) to the Colombian border where aid is being stockpiled on the Colombian side, in the city of Cúcuta.
“Confirmed – it’s rolling,” a spokesman for Mr Guaidó’s convoy told AFP news agency on Thursday after he left Caracas by car.
“We know that the regime is going to put all obstacles to prevent us from reaching the border, but nothing is stopping us, we are going to continue,” opposition MP Yanet Fermin told the news agency.
Meanwhile, the Venezuelan government announced it would deliver 20,600 of its own food boxes to the Colombian border area.
Battle of the bands?
British entrepreneur Sir Richard Branson has organised a huge benefit concert for Friday near the Tienditas Bridge crossing, on the Colombian side of the border at Cúcuta, to raise money for Venezuela.
Venezuela Aid Live, he says, was organised at the request of Mr Guaidó and another opposition leader, Leopoldo López.
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 which, according to AFP news agency, is just 300 metres away.