They were lanky and fresh-faced, and the battle they lost had been their first.

Packed into Ukrainian prison cells, dozens of captured Russian conscripts lay on cots or sat on wooden benches, wearing flip-flops and, in one instance, watching cartoons on a television provided by the warden.

In interviews, they recalled abandoning their positions or surrendering as they found themselves facing well-equipped, battle-hardened Ukrainian forces streaming across their border.

“We ran into a birch grove and hid,” said Pvt. Vasily, whose small border fort was overrun on Aug. 6 — at the outset of a Ukrainian incursion into Russia that was the first significant foreign attack on the country since World War II. The New York Times is identifying the prisoners by only their first names and ranks for their safety if they are returned to Russia in a prisoner exchange.

The fighting marked a significant shift in the war, with Ukrainian armored columns rumbling into Russia two and a half years after Russia had launched an all-out invasion of Ukraine.

Russia’s border, it turned out, was defended thinly, largely by young conscripted soldiers who in interviews described surrendering or abandoning their positions. Private Vasily said he had survived by lying in the birch forest near the Russian border for three days, covered in branches and leaves, before deciding to surrender.

“I never thought it would happen,” he said of the Ukrainian attack.

The Russian military command had, by all signs, made the same assumption, manning its border defenses with green conscripts, some drafted only months earlier. Their defeat and descriptions of surrendering in large numbers could increase Ukraine’s leverage in possible settlement talks and lead to prisoner exchanges.

It could also resonate inside Russia. The losses of young, drafted Russian soldiers during wars in Afghanistan and Chechnya stirred widespread discontent at home.

For the bloody trench fighting inside Ukraine, the Russian Army deploys older soldiers mobilized from their civilian jobs, volunteers and prisoners, who agree to a tour of duty in exchange for their freedom, should they survive. Conscripts are barred by law from being deployed outside Russia. But stretched for manpower, or blind to the risk, the military used them to guard the border.

In Russia, conscripts represent the bottom rung in a two-tiered military that took shape in the post-Soviet period. They are drafted after high school and typically serve a year performing menial tasks, like shoveling the snow at bases deep in Russia. Contract soldiers are volunteers, better paid, and they bear the brunt of wars in Syria and Ukraine.

Ukrainian officials say they have captured hundreds of young conscripts over the past week and a half, in what President Volodymyr Zelensky has praised as a move replenishing Ukraine’s “exchange fund” for the swapping of prisoners.

The prison in northern Ukraine, where the conscripts interviewed on Friday were held, has processed 320 prisoners of war so far, 80 percent of them conscripts, according to the prison warden, who asked not to be identified by name for security reasons; the numbers could not be independently verified. After processing, prisoners are sent west, farther from the fighting.

On Friday, the prison held 71 prisoners of war, packed into basement cells, where they are safeguarded from potential airstrikes, the Ukrainian military says.

They wore hand-me-down track suits, T-shirts and shorts that their captors had provided. Some had shrapnel or gunshot wounds. Wide-eyed, and appearing to be disoriented, they watched as guards escorted journalists into the cells for interviews.

The prison provided access for several media outlets, including The Times, whose journalists identified themselves, and asked permission for interviews and to take photographs. The detainees were interviewed after being captured, and in the presence of guards, so their accounts could not be independently verified. The guards did not intervene, and were some distance from the prisoners during the conversations.

The prisoners described being stationed at platoon strength, about 30 men, in concrete or earthwork fortifications spaced a mile or so apart along the border. There, they had faced a sudden, ferocious attack and quickly gave up the fight.

Ukrainian soldiers, independent military analysts and Russian military bloggers have also reported a quick rout along the border.

Pvt. Igor, a slender 21-year-old who was drafted in December, said Ukrainian artillery fire had picked up a few days before the incursion. “We reported to commanders, but they didn’t react,” he said. “They said, ‘Well, there’s nothing we can do about it.’”

On the day of the attack, bombarded by artillery, he tried to hunker down in the fortification, but it caught fire.

He and others ran for a nearby forest, he said. From his group of 12 who had tried to dash to safety, five survived, he said.

The position had only one recoilless rifle, he said, and it could not shoot at Ukraine’s American-provided Bradley infantry fighting vehicles as they attacked because it was pointed in the wrong direction.

Pvt. Sergei 20, from the region of Tatarstan in the Volga River valley, said his platoon leader had led the 28 soldiers from his fort on a chaotic retreat. They holed up in a village house, but Ukrainian soldiers found them. All surrendered.

The platoon leader, he said, had yelled out a window: “‘There are conscripts here. We want to surrender.’”

Some lamented being sent with inadequate training to a battlefield that has evolved into a complex mixture of exploding drones, electronic jamming and the use of finely honed tactics for storming trenches and fortifications.

Pvt. Dmitry 21, from the Komi region in northern Russia, said that when their radios stopped working on the day of the attack, he assumed it was because of Ukrainian jamming. Without orders, the soldiers realized they were overmatched and tried to retreat, but were captured. “They said we might see a sabotage group, maybe 10 guys, not an army,” he said.

While the Ukrainian forces took full advantage of the element of surprise and what both sides described as a weak and disoriented Russian defense, the Ukrainian advance has bogged down away from the border against better prepared Russian forces.

Conscripts are still being caught, however. This past week, the Ukrainian news media published a video said to show more than 100 conscripts lying face down in a field, reportedly filmed by the S.B.U., Ukraine’s domestic intelligence agency. The videos could not be independently verified.

For Mr. Zelensky, the influx of prisoners of war eases what had become a simmering domestic problem. The distraught families of Ukrainian soldiers held in Russia have staged weekly protests in Kyiv, the capital, to draw attention to their detained loved ones. Russia does not disclose the numbers of Ukrainians it has captured, but was assumed, before the incursion, to have more Ukrainians than Ukraine had Russians.

The haul of new prisoners has cheered these families.

That includes Tetyana Vyshnyak, whose son served with the Azov regiment and was captured and sentenced to 22 years in a Russian prison. There had been little hope of his release, given Russia’s advantage in the number of prisoners of war.

“For all of us, this is a great chance and hope that our loved ones will be exchanged,” Ms. Vyshnyak said in an interview.

Valeria Subotina, a former press officer with the Azov regiment, spent 11 months in Russian captivity before an exchange in 2023. “It’s incredibly difficult to go on living when your friends and loved ones are in captivity,” she said. “I hope the operation in the Kursk region can change this.”

Yurii Shyvala contributed reporting from northern Ukraine, and Stas Kozljuk from Kyiv, Ukraine.