"It's time that we be clear that, in the legislation, prison terms can be given and that we don't only have fines as punishment for buying sex," Fredrik Reinfeldt said in a speech in Stockholm.
In 1999, Sweden became the first country in the world to criminalize the purchase of sex. Although the law provides for up to six months behind bars, in practice judges so far have only imposed fines of up to 7,500 kronor ($1,140).
Similar legislation has since been adopted by Iceland, Norway, Slovakia, and soon France.
Reinfeldt said Sweden has convicted 4,782 people since its law came into effect, but that not one had gone to prison. His Moderate Party, he said, would campaign to incarcerate clients who knew the prostitute they used was a minor or a victim of human trafficking.
He said there were an estimated 400 to 600 prostitutes in Sweden who were there because of traffickers.