Venezuela has said that its border with Colombia has been partially closed, shortly after opposition leader Juan Guaidó defied a travel ban to cross it.
Vice President Delcy Rodriguez tweeted to say the "total, temporary closure" was due to serious threats against the country's sovereignty and security.
Tensions have been rising over a row about the delivery of humanitarian aid.
Two people were killed by Venezuelan security forces on Saturday near the border with Brazil.
The violence was condemned by the United States government, which said in a statement: "The Venezuelan military must allow humanitarian aid to peacefully enter the country. The world is watching."
Mr Guaidó, the leader of the country's opposition-dominated National Assembly, last month declared himself the country's interim leader.
He has since won the backing of dozens of nations, including the US. He has called the rule of President Nicolas Maduro constitutionally illegitimate, claiming that Mr Maduro's re-election in 2018 was marred by voting irregularities.
What happened on Friday?
Hundreds of tonnes of humanitarian aid sitting just outside Venezuela's borders have become a flashpoint between Mr Guaidó and President Maduro.
Mr Maduro has so far refused to allow the aid, which includes food and medicine, to cross over into Venezuela. Mr Guaidó has vowed that hundreds of thousands of volunteers will help bring it in on Saturday.
On Friday, rival concerts were held just 300m (980ft) away from each other on either side of the Venezuelan-Colombia border.
Mr Guaidó turned up at Venezuela Aid Live in Cucuta, organised by British businessman Richard Branson, on Friday.
He was greeted there by the presidents of Colombia, Chile and Paraguay - three of the nations who have recognised the 35-year-old lawmaker as interim president.
He alleged that he was able to cross over on Friday with the help of the Venezuelan armed forces. The claim is significant as President Nicolás Maduro has been able to retain power largely because of his military support.
Hours after his appearance, the announcement about the closure of bridges in Tachira state was made.
It follows a similar announcement made on Thursday about the closure of the border with Brazil - where another aid collection is being raised.
Violent clashes broke out there on Friday morning after members of an indigenous community reportedly confronted Venezuelan troops in the southern village of Kumarakapay.
Witnesses said that troops opened fire on individuals who tried to block a road to prevent military vehicles from passing.
Human rights campaigners said soldiers shot and killed two people and wounded 15 others.
A spokeswoman for UN Secretary-General Antonio Guterres said he had a meeting with Venezuelan Foreign Minister Jorge Arreaza on Friday in New York, in which he urged authorities to refrain from using lethal force against demonstrators.
Why is the aid delivery contentious?
Economic conditions have deteriorated rapidly since President Maduro took power in 2013.
The UN says about three million people have fled the country over the last few years.
Hyperinflation has caused the cost of essentials to soar, leaving many unable to afford basics like food and medicine.
Mr Guaidó has said the aid deliveries are necessary to stop Venezuelans dying. He has vowed to get his supporters to mobilise en-masse to get it in on Saturday.
'Deeply uncertain' day ahead
Analysis by Katy Watson, BBC News South America correspondent
This is the day Venezuela's opposition has been waiting for. A day that will test the loyalty of the country's armed forces towards Nicolas Maduro and determine his future.
Lorries laden with aid are expected to set off from both Colombia and Brazil and attempt to cross the border. A ship carrying aid is also travelling from Puerto Rico.
Throughout Venezuela, people will gather at military barracks to ask soldiers for their help in the aid effort.
Until now, senior officers have remained loyal to Mr Maduro - but with pressure being heaped on them to help the Venezuelan people, will they listen to their leader or change sides, support Juan Guaido and open the borders? These next few days are deeply uncertain.
President Maduro denies there is any crisis and has branded the aid plans a US-orchestrated show.
Performers at his rival concert on Friday performed in front of a backdrop that said #TrumpHandsoffVenezuela, the AFP news agency reports.
The US President has led the effort to recognise Mr Guaidó as president, and has implemented economic sanctions to put pressure on President Maduro's government.
At a speech earlier this week, he urged the Venezuelan military to switch sides and abandon their support of the president.
He has repeatedly reiterated that "all options are open" in regards to the US response to the unfolding crisis.