PORTLAND, Ore.—Files kept secret for decades that detail hundreds of claims of child sexual abuse by troop leaders of the Boy Scouts of America are at the center of a civil court case that began here Wednesday.

Lawyers for a victim say the files show a centralized national effort to conceal abuse while lawyers for the Boy Scouts say the files demonstrate proactive efforts to stop it. The group has acknowledged that abuse occurred.

The lawsuit, brought by a man who was abused by a scout leader in Oregon in the early 1980s, seeks $4 million in damages for the victim, who was about 12 at the time. Timur Dykes, a former scout leader and convicted pedophile, has admitted to abusing the man, Kerry Lewis, 37, when he was a boy.

Scout leaders have been found guilty of sexual-abuse crimes in various cases across the country for more than two decades.

On Wednesday, lawyers for Lewis said in court that they would make a broader case, bolstered by the recent release of about 1,000 individual files kept by the Boy Scouts of America from 1965 to 1984. The lawyers said they would argue that the national group was aware of abusive leaders across the country, including Dykes, and frequently took no action to prevent more abuse.

“;The Boy Scouts of America actually set back the child abuse prevention movement in this country, held it back, because of their secrecy,”; Kelly Clark, a lawyer for Lewis, said in opening arguments here.

Parts of the files have emerged in other lawsuits, and officials with the Boy Scouts, based in Irving, Texas, have previously acknowledged concealing some abuse. But in court on Wednesday, Chuck Smith, a lawyer for the Boy Scouts, said the files—sometimes called “;the red-flag file”; or the “;confidential files”;—showed that the group had been ahead of national trends in tracking abuse, and he said the files had been kept confidential only to protect victims.

“;We do not produce them willingly or voluntarily for a very good reason until by a court order,”; Smith said in opening arguments.

Clark and Paul Mones, the other lead lawyer for Lewis, have led several high-profile child abuse cases, including against the Boy Scouts, the Roman Catholic Church and the Church of Jesus Christ of Latter-day Saints.

Clark said the files, which were not made public on Wednesday, showed that national and regional scout leaders had allowed troop leaders and volunteers to work with scouts for years after complaints arose that they had abused children, sometimes even after they had been convicted of sexual abuse. He said evidence would show claims of abuse as early as the 1920s.

In a statement, Deron Smith, the group's national spokesman, said the Boy Scouts of America had aggressively addressed the issue of abuse. “;Unfortunately, child abuse is a societal problem, and there is no fail-safe method for screening out abusers,”; the statement said. “;However, the BSA has some of the oldest and most respected youth protection measures of any youth-serving organization.”;

The jury trial, in Multnomah County Circuit Court, is expected to last more than a week.