Planned Parenthood said its website was taken down Wednesday afternoon in what it called an "extremist" attack.
The group said the site was taken down through a distributed denial of service. That's when hackers flood a site with traffic to keep users from accessing it.
No one has claimed responsibility for the hack.
As of Thursday morning, a message saying the site was attacked was still on the homepage.
RELATED: Covert video targets Planned Parenthood fetal-parts policy
The incident comes following the release of several undercover videos that appear to show Planned Parenthood officials discussing the sale of fetal tissues and organs.
The footage, which is highly edited, was released by anti-abortion activists.
Planned Parenthood says its website was taken down in 'attack by extremists'