WASHINGTON — New opponents confronted Facebook on Wednesday as it moves forward with a plan to encrypt all of its messaging platforms: child welfare advocates who said that encryption would allow child predators to operate with impunity across the company’s apps.

“Facebook has a responsibility to work with law enforcement and to prevent the use of your sites and services for sexual abuse,” a group of 129 child protection organizations, led by the National Society for the Prevention of Cruelty to Children, said in a letter to the Silicon Valley company. “An increased risk of child abuse being facilitated on or by Facebook is not a reasonable trade-off to make.”

The letter indicates how activists and law enforcement agencies have seized on child exploitation as a new way to combat the expanded use of encryption in consumer technology.

The Justice Department and its counterparts in Britain and Australia previously used the threat of terrorist activity to rail against encryption, saying that tech companies were shielding malicious and dangerous criminals. But they have recently shifted their focus to child exploitation as tech companies have made good on plans to make it harder to see or stop illicit activity on those platforms.