By Agence France-Presse
Visitors to the site and its Canadian version backpage.ca were shown a notice by various federal agencies declaring they had seized the domain.
The notice said: "backpage.com and affiliated websites have been seized as part of an enforcement action by the Federal Bureau of Investigation, the US Postal Inspection Service, and the Internal Revenue Service Criminal Investigation Division, with analytical assistance from the Joint Regional Intelligence Center."
It added that additional information would be forthcoming at 6:00 pm (2200 GMT).
Backpage.com is accused by its critics of being the biggest website for prostitution in the world, with its classifieds used to promote paid-for sex with minors and the victims of human trafficking.
The US Congress passed a bill last month aimed at countering sex trafficking that allows victims of such exploitation to seek justice against website owners who knowingly promote or facilitate the practice.
Supporters hailed the passage of the Stop Enabling Sex Traffickers Act (SESTA) as a major victory, but the measure is not without its critics: some say it would undermine a basic underpinning of the internet, which enables websites to host information from third parties without liability.