The 65-year-old and 13 co-defendants are on trial accused of aggravated pimping in connection with a sex ring centred on the Hotel Carlton in Lille.
In his first testimony since the trial began Feb. 2, Strauss-Kahn reaffirmed his long-standing defence that he did not know of the "prostitutional character" of the women who took part in his orgies.
Strauss-Kahn's arrival at the courthouse in Lille was disrupted by three topless protesters from the provocative group Femen, who were detained by police.
Strauss-Kahn's chances of becoming French president were ruined by a separate sex scandal in New York.
The economist, known widely as DSK, faces up to 10 years in prison and a 1.5 million-euro ($1.7 million) fine if convicted.
Strauss-Kahn and the 13 other defendants are accused of operating a prostitution ring out of luxury hotels in Paris, Washington D.C., Lille and Brussels.
"I had a very hectic life, with just a few outlets for recreation, and these sessions were part of that," Strauss-Kahn told the court.
Adding that he believed the women to be "a group of friends" Strauss-Kahn said that if he'd known they were prostitutes "I would have totally stopped participating in these soirees."
Two of Strauss-Kahn's co-defendants also testified Tuesday that they'd kept hidden from Strauss-Kahn the fact that they'd hired prostitutes for the orgies. "It was a secret between him and me," Fabrice Paszkowski told the court, referring to fellow co-defendant David Roquet.
The court has so far heard testimony from some of Strauss-Kahn's fellow defendants, who include a Belgian brothel owner, local businessmen, a police officer and hotel staff accused of organizing sex parties for Strauss-Kahn's benefit.
Investigators have compiled hundreds of pages of testimony from prostitutes describing the orgies.
One of the prostitutes, called Mounia in court, testified that while she never discussed payment with Strauss-Kahn, everyone involved knew she was a prostitute. "For me it was clear that I was there as a prostitute," she said.
It's not illegal to pay for sex in France, but it is against the law to solicit or to run a prostitution business.
Prostitutes questioned in the case have said that between 2009 and 2011 — when the IMF chief was dealing with a global financial crisis — Strauss-Kahn was organizing orgies at luxury hotels in Paris, at a restaurant in the French capital and also in Washington.
Hundreds of reporters are covering the trial.
Keller reported from Paris.
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