Leonardo Sánchez wailed in pain for hours on the deportation flight from Louisiana to West Africa, he and six other detainees said. He’d been cuffed around his wrists and ankles. A chain around his waist dug into his grossly protruding abdomen — the result of years-old wounds and surgeries that doctors said left his intestines barely inside his body. Mr. Sánchez, a 49-year-old Cuban, begged for medical attention from several of the Immigration and Customs Enforcement agents on the aircraft. “I was in pain, crying like a little kid,” he said.

But the agents wouldn’t remove his chains or let him use the toilet, he and other detainees said. He wet himself. Like many of the more than two dozen deportees on the flight with him, Mr. Sánchez said he did not know where he was being taken.

Several panicked when they suspected it was Liberia, after one of them saw it written on a sheet of paper held by an ICE official in Louisiana. When some protested, they were beaten by agents, five of the deportees said. Some were slammed to the ground or struck with blows, several said.

One showed cuts along his wrists and bruises around his neck, which he said he received after resisting getting on the plane. Another said some deportees were left screaming in pain. The planeload of deportees, which arrived in Liberia this month, carried the first of 1,200 people expected to be sent to the West African country by the United States over the next year.

The arrangement is one of the largest in a growing number of deals the Trump administration has made to send deportees — including people with court orders protecting them from being sent back to their home countries — to nations halfway across the world where they have no ties. In recent weeks, the White House has pushed to expand these so-called third-country deportation efforts and boost the number of people expelled from the United States, according to a U.S. official. In July, ICE arrests jumped to nearly 50,000 people, and the administration has focused on deporting people who have been able to remain in the country for years with court orders protecting them from being sent home, another official added.

The New York Times interviewed 10 of the deportees on the flight to Liberia, all of whom described a harrowing journey that included months in detention in the United States and a panicked 14-hour expulsion to Africa, in which agents wrestled and shackled frightened detainees. Several spoke on the condition of anonymity for fear of reprisals. All of them said they experienced or witnessed what they described as abusive or violent treatment by ICE officials during the course of their deportation or while in detention in the United States. “They didn’t treat us like people, like human beings,” said Carlos Téllez-Sanchez, a deportee from Venezuela who was expelled to Liberia. “They can say or do anything to you and there’s nothing you can do.” The White House, Department of Homeland Security and State Department said in statements that the only deportees being sent to third countries are those who cannot be returned to their home nations or refuse to go back. “As Secretary Rubio has said, we remain unwavering in our commitment to end illegal and mass immigration,” said Tommy Pigott, a State Department spokesman.

When the flight arrived in Liberia’s capital, Monrovia, Mr. Sánchez, the Cuban national, and three others said they refused to disembark, afraid of being abandoned in an unfamiliar country. They said they were forced onto the tarmac by ICE officers who yanked and twisted their handcuffs, pinned them to the ground and pressed their knees directly onto their necks. Mr. Sánchez said the federal agents knocked him onto the ground and knelt on his bulging abdomen, leaving him in agony, his pants still damp with urine. Other deportees began shouting, “‘You abuser! If he dies, it’s on you!’” Mr. Sánchez and three other deportees said.

The Department of Homeland Security said that no detainees were refus...