The general counsel for Craig Ferrer, chief executive of small ads website Backpage, has said the charges against him made by the California and Texas attorneys general are “illegal”.
He and two of the site’s controlling shareholders are accused of pimping.
Allegations that the site has carried adverts aiding forced prostitution have dogged the platform for some years.
Liz McDougall, representing Mr Ferrer, told the BBC the prosecution was an “election-year stunt”.
“The actions of the California and Texas attorneys general are flatly illegal.
“They ignore the holdings of numerous federal courts that the First Amendment protects the ads on Backpage.com,” she said in a statement.
“The actions of the attorneys general also violate Section 230 of the Communications Decency Act pre-empting state actions such as this one and immunising web hosts of third-party created content.”
The BBC has contacted the office of the California attorney general for comment.
Adverts offering prostitution services were against the website’s policies and were blocked, and the site removed them when contacted by law enforcement, Ms Mcdougall added.
Backpage operates in hundreds cities around the world, including London, Jerusalem, Beijing and Miami.
The State of California Department of Justice conducted a three-year investigation and concluded that “many” of the site’s adult escort adverts involved prostitutes and victims of sex trafficking, including children.
“Raking in millions of dollars from the trafficking and exploitation of vulnerable victims is outrageous, despicable and illegal,” said Attorney General Kamala D Harris, from the State of California Department of Justice.