At least one person has died and several others have been injured in clashes at Venezuela’s border with Brazil, local media report.
Venezuelan security forces opened fire on a group of people who tried to block them from travelling to the border.
President Nicolás Maduro has closed the country’s border with Brazil amid a row over the delivery of humanitarian aid.
It comes as two rival concerts get under way on either side of a bridge linking Venezuela and Colombia.
An event held on the Colombian side is aimed at raising money for Venezuela, where the opposition says the lack of basic items has left thousands at risk of dying.
Meanwhile, President Maduro’s government is holding its own concert.
Humanitarian aid for Venezuela has been arriving at the borders of neighbouring Colombia and Brazil.
Opposition leader Juan Guaidó and his allies hope to collect the relief gathered there on Saturday, in defiance of President Maduro, who denies any crisis and calls the aid plans a US-orchestrated show.
President Maduro has said he could also shut the border with Colombia to stop the opposition bringing in aid.
Why are the concerts taking place?
The concerts, being held just 300m (980ft) apart, represent two opposing sides of a power struggle between President Maduro and Mr Guaidó, who declared himself interim leader last month and is recognised by dozens of countries, including the US and most Latin American nations.
One of the concerts, Venezuela Aid Live, is organised by British entrepreneur Sir Richard Branson, who said Mr Guaidó had asked him to set it up.
It is being held near the Tienditas bridge crossing on the Colombian side of the border at Cúcuta. Organisers invited 32 artists including Latin stars Rudy Mancuso, Juanes and Despacito singer Luis Fonsi, and Swedish DJ Alesso.
A few celebrities – including Venezuelan-American singer and actress Lele Pons and Venezuelan singer Danny Ocean – and politicians have also confirmed they were taking part.
“This concert is a humanitarian concert; every country, all people in the world want freedom,” Mr Branson said.
“Anybody that does anything for the right reasons is always going to get criticism. The positive thing is that 99% of people around the world are embracing what is going on here today.”
Following news of the event, President Maduro promised to hold a rival concert on the Venezuelan side of the bridge – a three-day festival which has been dubbed Hands Off Venezuela.
The government still has not announced an official line-up for Hands Off Venezuela, but unconfirmed reports claim that about 150 artists are taking part.
What’s going on at Venezuela’s borders?
Brazil, which recognizes Mr Guaidó as Venezuela’s legitimate leader, had previously vowed to make humanitarian aid available in the Brazilian city of Pacaraima, to be collected by opposition supporters.
There are already planes and lorries carrying tonnes of US aid parked on the Colombian side of the Tienditas bridge, and President Maduro has refused to allow it into Venezuela, saying it is part of a ploy by the US to invade the country.
Despite denying that there is a humanitarian crisis at all, President Maduro reportedly received a shipment of aid from Russia earlier this week, according to Russian state media.
So how will the aid get in?
It is not yet clear. Mr Guaidó has said he plans to get aid into the country on Saturday by urging Venezuelans to mobilise en masse and form “caravans” and a “humanitarian avalanche” at the borders.
But even with this effort, it is uncertain whether or not aid will be allowed in.
A spokesman for Mr Branson told the BBC that he was working with the Colombian entrepreneur Bruno Ocampo to organise the concert and sort out the logistics while Mr Ocampo said the details “remain confidential”.