Venezuelan soldiers have blocked a bridge on the border with Colombia ahead of a shipment of humanitarian aid, the opposition says.
The aid is being arranged by opposition leader Juan Guaidó, who declared himself interim president last month.
But it is not clear how the aid will be distributed. President Nicolás Maduro, who has the support of the army, has rejected letting it into the country.
Venezuela is in the grip of a worsening humanitarian crisis.
Mr Guaidó is head of Venezuela’s National Assembly and says the constitution allows him to assume power temporarily when the president is deemed illegitimate.
He has secured the backing of over 40 countries, including the US and most Latin American and European nations. Mr Maduro still has the support of China and Russia.
What is happening at the border?
Military officers used a tanker truck and a cargo container to block access to the Tienditas bridge, which links Cucuta, Colombia to Urena, Venezuela, opposition politician Franklyn Duarte told AFP.
“Troops from the armed forces are blocking the pass”, he said.
Other opposition leaders appealed to the military to allow aid trucks to cross the border.
“You know there’s a red line, you know well there’s a limit, you know that medicines, food and medical supplies are that limit,” said lawmaker Miguel Pizarro.
Pictures shared on social media appeared to show residents confronting military officers in Urena.
Mr Maduro said humanitarian aid would be the start of a US-led invasion, insisting that “no one will enter, not one invading soldier.”
How will the aid scheme work?
Mr Guaidó does not control any territory in Venezuela so, instead, he is planning to set up collection centres in neighbouring countries where Venezuelans have fled to.
He said he wanted to set up an international coalition to gather aid at three points, and press Venezuela’s army to let it into the country.
In a tweet on Sunday, US National Security Adviser John Bolton said plans were being advanced.
In his State of the Union speech, he reiterated his support for Mr Guaidó saying “we stand with the Venezuelan people in their noble quest for freedom.”