SEOUL -- -- For weeks, they have formed a black wall of defense around South Korea's regime: row upon row of immovable riot police, faceless behind their black helmets, gas masks and shields.

In recent days, however, cracks have appeared in the wall, revealing tension in the closed world of what may be the globe's most formidable nonlethal fighting force.

On Monday, a 22-year-old riot policeman who allegedly disobeyed an order was punched to death on a police bus by a senior officer, authorities reported Wednesday. Human rights officials here said it was not the first instance of riot police being killed or tormented into suicide by superiors.

At 3:30 a.m. Wednesday, a 23-year-old riot policeman who was drafted into the force two years ago deserted his post and turned up at a church human rights office, saying he had come to protest government policy. Officials here said it was the first such desertion ever.

Late Wednesday night 70 to 80 policemen stormed into the office looking for the dissident officer, a church worker said. But the officer, Yang Seung Kyun, had slipped away minutes before.

Yang said in an interview Wednesday afternoon that, like him, many of his fellow officers have little stomach for fighting students who are their contemporaries, their friends or -- in at least one case -- their brothers.

"Soldiers are supposed to be serving the country and protecting the people," said Yang. "Instead, we are the front lines of their repression."

On June 29, after weeks of battles between riot police and demonstrators, the government stunned Koreans by agreeing to most opposition demands, including direct presidential elections and release of most political prisoners. Yang said he had decided to desert before the announcement, and carried out his plan anyway because he does not believe the authorities are serious about democratization.

Whatever his motivation, Yang's description of life in the combat police barracks, along with the sketchy tales of beatings and suicides, provides a rare inside glimpse of what outwardly seems a perfectly disciplined force.

Although demonstrations largely ceased when the government promised reform, riot police have been called into action at Yonsei University here almost every day this week. Yonsei students have been protesting the death of a classmate who was killed by a tear-gas grenade during a June 9 rally.

It is a testament to the discipline of the combat force that it killed no other demonstrators during 20 days of turbulent clashes in June (one demonstrator died when he fell off a bridge, and one policeman was run over by a bus). Fearsome as they look behind their helmets, the riot police are virtually unarmed by U.S. standards.

While comrades behind them lob tear gas at the students, and demonstrators in front of them throw stones, the front ranks of troops often are caught in the middle. Ordered to act as a human wall, they have only their shields to fend off rocks and blows from stick-wielding students.

Units of plainclothes toughs wearing hard hats sometimes wade into the throngs, beating students with clubs and grabbing those they can catch. But the uniformed riot police do not follow suit.

Yang said the "blue jean units," as he called the plainclothesmen, are a mystery even to the rank-and-file. They live in separate barracks and report to different commanders. Their esprit de corps is said to be much higher, and persistent rumors -- and one credible official source -- say some are recruited from the ranks of petty criminals.

Combat police, on the other hand, are young men who join to avoid being drafted into the Army or, as in Yang's case, are transferred from the Army to do their national service with the police.

When police slide their helmets off, some of their faces reveal unhappy draftees. Blinking back tear gas, sweating from padded uniforms, they slump in formation for a cigarette break between skirmishes. Many are former university students with friends or acquaintances on the other side.

One Yonsei freshman said she knows an older student whose brother is in the police force. One afternoon, both were deployed.

"The policeman could see his brother through the helmet," she said. "But the student couldn't tell which policeman was his brother."

Still, she added, the student hurled rocks as always.

"I feel sorry for the policemen, having to wear such heavy clothes in such hot weather for hours and hours in the tear gas," she said. "But during the demonstrations, we really feel they are monsters, and they probably feel we are monsters, too . . . I can see why they despise us."

Yang said that in some cases police commanders stir up sentiments against the students, "raging" at them during training sessions.

But in recent days, as the force has become increasingly worn down, some officers have become slow to lob tear gas or join the battle when ordered. As a result, abuse by superior officers has become more common, according to Kim Dong Wan, director of the Human Rights Committee of the National Council of Churches in Korea.

"A lot of them may be tired, they act slowly, and the police overreact," he said.

In the most recent such incident, an officer accused Cho Yang Kun, 22, of lacking discipline and repeatedly punched him in the chest. Cho was pronounced dead soon after.

The incident occurred on a police bus while 40 other policemen watched, Reuter reported Wednesday. A Home Ministry official who asked not to be identified said the government will prosecute the senior officer who is the alleged assailant.

Yang said brutal beatings are common, a tool to instill "unconditional obedience."

In February, a human rights worker said, another policeman was beaten to death by two senior officers for "not listening well, not having the right spirit."

In addition, according to the church office, since September there have been three suicides and one attempted suicide by riot police who reportedly had been beaten, hazed or, in one case, had money extorted by senior officers.

Meanwhile, Yang could face a 10-year sentence for leaving his post, officials said.

"Normally, the punishment for people who go AWOL for a time is detention or loss of family visits," Yang said before he left the office. "In my case, I cannot imagine what will happen."