SAN CRISTOBAL, Venezuela — At Tachira Central Hospital, ceilings are caving in, most ambulances don’t work, and antibiotics are scarce. Now harried doctors are stockpiling blood and drafting weekend medics as Venezuela braces for what the opposition is calling the “D-Day” of humanitarian aid.

“This could turn into a dangerous conflict: the armed forces versus the people,” said Laidy Gómez, the opposition governor of Tachira, a Venezuelan state abutting Colombia. She has ordered state hospitals to prepare for casualties Saturday, when, in defiance of President Nicolás Maduro, an army of volunteers will seek to break the socialist government’s blockade of international relief.

“It would be a crime against humanity to act against thousands of people who are clamoring for food and medicine,” Gómez continued. “But I’m worried that Nicolás Maduro is looking for a fight.”

Maduro on Thursday ordered the closure of the border with Brazil and weighed sealing the border with Colombia, not far from this western metropolis, as his government scrambled to respond to the planned Saturday operation. Venezuela’s National Institute of Civil Aviation issued an order grounding private jet traffic nationwide. Commercial flights were still operating, though Air France said it would cancel flights to Caracas through Monday, given the heightened tensions.

In an apparent bid to counter international criticism of turning away the aid — provided by the United States and other countries advocating for Maduro’s ouster — Maduro’s vice president, Delcy Rodríguez, said the government on Thursday had sent the United Nations a list of medicines the country needed for “humanitarian assistance.” Maduro also announced that 7.5 tons of medical supplies had arrived Thursday from Russia and the Pan American Health Organization.

Maduro’s directives came as the U.S.-backed effort to topple his government is entering a critical and potentially more dangerous phase.

A month after opposition leader Juan Guaidó declared Maduro a usurper and claimed to be Venezuela’s rightful leader, the government’s enemies were in the midst of a risky gambit. By bus, car, boat, plane, motorbike and foot, thousands of Venezuelans, including Guaidó, were already mobilizing and heading toward the borders. Their plan: to force open Venezuela’s doors by force of numbers.

Mariela Magallanes, a Venezuelan politician, said Feb. 22 that her bus was attacked while she and other lawmakers were traveling to the border to pick up aid.

With Monday also marking the deadline for the Trump administration and the Maduro government — which last month officially broke diplomatic ties — to reach agreement on keeping a handful of diplomats in their capitals, Venezuela was confronting a crisis on multiple fronts. Failure to reach even a temporary deal could escalate tensions when Washington has threatened military intervention and is working with the opposition to coordinate Saturday’s operation.

“This message is for the Venezuelan military: You will ultimately be responsible for your actions,” Adm. Craig Faller, the head of U.S. Southern Command, said this week after a meeting with the leader of Colombia’s armed forces. “Do the right thing. Save your people and your country.”

Vice President Pence is expected to be in Colombia on Monday for a gathering of Latin American leaders where, his office said, he would “voice the United States’ unwavering support for interim President Juan Guaidó and highlight the Venezuelan people’s fight for democracy over dictatorship.”

The opposition relief plan, which involves flotillas in the Caribbean and caravans through the Andes and the Amazon, is being hailed as a way to ease spreading hunger and disease in a nation on the verge of becoming a failed state. Yet the government’s enemies also have another, more political purpose: to use the humanitarian operation to trigger Maduro’s downfall.

They are calculating that rank-and-file members of the military and other security forces will not fire on unarmed civilians trying to cart boxes of aid across the border. Should the military disobey orders to stop volunteers, they believe, it could rob Maduro of his key source of power: the threat of brute force to keep the nation in line.

If the security forces or pro-government militias resort to deadly force, it could spark a more direct confrontation with the Trump administration.

“There will be an international response if the armed forces fire on the people,” said Eduardo Delgado, 37, an opposition leader who hopes to marshal as many as 40,000 volunteers at the border. “And the U.S. is leaving no option off the table.”

Guaidó and his team in a caravan of 10 vans were headed Thursday toward San Cristobal, and ultimately to the Colombia border. Four buses traveling ahead of him with opposition lawmakers, journalists and volunteers were stopped in the state of Carabobo by national guardsmen throwing tear gas, said Roberto Campos, an opposition lawmaker who was on one of the buses.

Some of the lawmakers struggled with the guardsmen, who, Campos said, sought to take their IDs. A Guaidó spokesman confirmed that his vehicle was still making its way west.

“They’re not letting us off the bus right now,” Campos said. “They’re giving us no information, and we don’t know what’s going to happen.”

Maduro’s government deems the aid operation a Trojan horse invasion by the United States and has ordered the severing of sea and air links with three Caribbean islands being used as staging grounds for aid. It also dispatched military reinforcements and warned of “cadavers” on the borders. The bulk of the aid is in the Colombian border city of Cucuta, where the opposition’s allies, including billionaire Richard Branson, are hosting a benefit concert for Venezuela on Friday.

“We have the armed forces deployed across the nation,” Maduro said Thursday in a televised meeting with his top brass.

Colombian officials countered that they were considering taking down border fences to allow Venezuelan volunteers — who were being encouraged to dress in white — to freely enter. Either way, Venezuelan opposition officials said they would start their attempt to move aid at 9 a.m. Saturday and would form human chains and carry out illegal crossings if the Maduro government blocked their way.

“We know what we are confronting,” said Gaby Arellano, a Venezuelan opposition deputy speaking Thursday in Cucuta.

As the opposition sought to put its plan into action, San Cristobal, Venezuela’s largest metropolis near the Colombian border, was poised to be one of the first to receive humanitarian aid.

A once-wealthy hub of agribusiness, the city of nearly 300,000 now stands as a textbook case of Venezuela’s need.

Grocery stores selling at regulated prices were out of chicken, beef and flour this week. Those offering goods at free-market rates were well-stocked, but locals could not afford the exorbitant prices.

“My son needs humanitarian aid,” she said. “We all need humanitarian aid. The government should not block it. . . . I cry every night because I’m worried I won’t be able to pay for the pills.”

Yet the humanitarian aid operation is unlikely to offer the assistance the region’s largest hospital needs most.

“Humanitarian aid is wonderful, but it’s not going to solve our biggest problems,” said Renny Cardenas, the hospital’s director.

In a country resting on the world’s largest known oil reserves, gasoline shortages are so bad that lines at stations can stretch for more than a mile.

The shortages are partly due to fuel smuggling, but they will only become worse nationwide because of oil sanctions imposed by the United States.

“The sanctions are a measure to pressure the government, and of course we all know it’s going to be hard,” said Alfredo Ramirez, 31, speaking from his red pickup, after spending 24 hours in a gas line. “But we want the government to hand away power. This country is lost right now.”

Mariana Zuñiga in San Cristobal, Andreina Aponte in Caracas and Dylan Baddour in Cucuta contributed to this report.