Venezuela braces for aid deadline amid rival concerts on border; one held by Richard Branson

CUCUTA, Colombia -- Venezuelan opposition leader Juan Guaido joined thousands of people on Colombia's border with Venezuela Friday for a blockbuster concert backed by British billionaire Richard Branson that aimed to raise money to alleviate suffering in Venezuela. The Venezuelan regime planned a competing concert across the border.

The dueling concerts come as Venezuela's opposition braces for a Saturday ultimatum that its leader, Guaido, has set for aid to enter the country. On Friday, tons of humanitarian aid sat in a warehouse in the Colombian border city of Cucuta, as Venezuelan President Nicolas Maduro continued to block it from entering the country and a top U.S. envoy, Elliott Abrams, was due to arrive with more U.S. government aid.

In Cucuta, concert-goers gathered for Branson's Venezuela Aid Live concert, which featured over 30 of the most renowned Latin American artists, all of whom volunteered to perform at the concert without compensation, according to organizers. Many of the artists are Venezuelan themselves.

Guaido made a surprise appearance at the event later in the day, wading through the crowd after defying a travel ban imposed on him by Venezuelan authorities.

The concert was free to attend and took place at the Tienditas bridge between Colombia and Venezuela, which is near the stockpiled humanitarian aid. The stage was set only a few feet away from where Maduro stationed containers and military personnel on the bridge to block access to Venezuela.

When asked on Friday about criticism that the concert and the humanitarian aid were being politicized, Branson, who organized the concert, told reporters that when people do things with good intentions they will always be criticized no matter what.

"The good thing is that 99 percent of people around the world are embracing what’s happening here today. And for those people who think Venezuela is a utopia, that nobody is suffering, they really should come here today and go into the crowd of people here and talk to all the Venezuelans and ask them why they’ve left their country," Branson said. "Something must be wrong. And so, for the doubters, I would ask them to talk to Venezuelans who’ve had to leave Venezuela to see the horrors that are happening.”

Branson said he had been inspired by Guaido's call "for the world to help" Venezuelans.

"We need to try to help those who are not getting medical help, help those who are hungry," Branson said. "And today, hopefully, on the back of this concert, we can start maybe getting supplies into Venezuela so that people are not suffering so much."

Abrams, a U.S. special envoy, accompanied one last shipment of humanitarian assistance from the United States to Cucuta on Friday, a day before the deadline. Nearly 210 tons of aid have been stockpiled there over the last couple of weeks.

Abrams spoke to the press briefly prior to his departure, calling on Venezuela’s military to let aid in on Saturday, when Guaido, who has declared himself Venezuela's interim president, and his supporters plan to begin bringing that aid back across the border to a country starved of food and medicine. The U.S., Colombia and 50 other countries have recognized Guaido as the country's rightful president.

Maduro on Thursday ordered the Brazilian border closed, threatened to do the same on the Colombian side and blasted the U.S. for what he said was using aid to undermine him.

Abrams told reporters on Friday that onboard the American military plane there was enough rice to feed 2,000 people for one month, along with medical supplies including wheelchairs, crutches, bandages and exam gloves.

While Secretary of State Mike Pompeo said the U.S. will help in Saturday's effort, Abrams made no mention of an American role. Officials at the U.S. Agency for International Development, or USAID, have told ABC News the U.S. has done its part, and that it’s now up to Guaido to get the aid in.

Abrams told ABC News that members of the Venezuelan army would cross a red line if they shot their fellow citizens.

"They have instructions to stop people from getting humanitarian aid," Abrams said. "This is their moment to say, 'No, we’re not going to do that to our fellow citizens."

An official with USAID, which has spearheaded humanitarian assistance for the Venezuelan refugee crisis for a year now, wouldn’t say whether the U.S. expects violence or not.

“We’ll see what happens, but you can definitely count on a response from the U.S. government” if there is, the official told ABC News.

“The U.S. monitors everything that’s happening in Venezuela," the official added. "The appropriate authorities will figure out what actions they want to take.”

Guaido has said he and tens of thousands of supporters will attempt to move that aid into the country on Saturday.

Meanwhile, in Venezuela on Friday, near the country's border with Brazil, one woman died and a dozen others were injured amid clashes between an indigenous group and the Venezuela National Guard and army, which was moving tanks to the Brazilian border, the Associated Press reported.

In Cucuta, Abrams met with Colombian President Ivan Duque and other regional leaders as they assemble more aid and tour facilities.

In Washington D.C., representatives for Guaido said they hoped the Branson-backed concert would raise $100 million for Venezuelans.

ABC News' Ben Gittleson contributed reporting.