WASHINGTON — Planned Parenthood on Monday told congressional investigators that abortion opponents had harassed and unlawfully infiltrated its clinics for years and most likely possessed thousands of hours of surreptitious video recordings that they could “deceptively edit” and spread for months to come.

The charge came in a five-page letter from a Planned Parenthood lawyer that was the group’s initial response to an investigation by Republicans on a House committee. Last week, the committee opened the inquiry after anti-abortion activists circulated a video showing one of the organization’s doctors describing how some affiliates provide donated fetal tissue to researchers.

The activists allege that the video proved that Planned Parenthood sells parts from aborted fetuses for profit, which is a criminal offense — an allegation now echoed by some Republican lawmakers and presidential candidates. Planned Parenthood denies the charge, citing the doctor’s statements in the video as proof that affiliates charge only small amounts to cover handling costs, which is legal.

The group’s prediction of more videos to come is based on reports from its clinics nationwide that abortion opponents had infiltrated them, posing questions to unsuspecting employees and patients. If more videos surface, that could galvanize conservatives in Congress for some time, even contributing to the risk of a government shutdown this fall if enough Republicans try, as they have in the past, to block spending bills unless federal funding to Planned Parenthood is eliminated.