If convicted, Mr. Strauss-Kahn faces up to 10 years in prison and fines of more than €1.5 million. He denies any criminal offence.
Three topless feminist protesters, with slogans painted on their bodies, jumped on to Mr Strauss-Kahn's car as it approached the court and they had to be forcibly removed by the police. One of the women had "pimps, clients, guilty" painted on her chest.
The one-time Socialist frontrunner in the 2012 French presidential election also said his contacts with Fabrice Paszkowski, a businessman accused of supplying prostitutes for group sex, paying them and travelling with them to meet the former IMF boss, were infrequent. "I had other things to do," he said in a letter read out to the court. "[I was] saving the world . . . I had political ambitions."
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Mr Strauss-Kahn's composure began to break down following the testimony of a former prostitute, known as Mounia to protect her privacy, who said she attended one of the evenings with all four men and had sex with Mr Strauss-Kahn.
She choked down tears as she was drawn into giving a detailed account of her sexual encounter with the man known as DSK, describing the "brutal" nature of the sex and "unnatural practices I had never experienced before".
Mr Strauss-Kahn began biting his lip, looking at his watch and talking to his lawyer as she described him engaging in anal sex during an encounter when she was alone with him in a bedroom: "It was brutal but consensual until the end because I needed the money," she said. "He saw that it was not what I wanted. I cried."
"There were activities . . . against nature. I wasn't used to doing things of that nature," said the woman, speaking softly and hesitantly. "It wasn't violence, it was a relationship of force."
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"He was smiling from the beginning until the end."
Mounia said she never discussed money with Mr Strauss-Kahn, however, and was paid afterwards.
Testimony by escort girl identified as "F", who was also present the night with Mounia, was read aloud in court. She said Mr Strauss-Kahn was "very attentive" and "never brutal", adding that "it wasn't clear" the girls were prostitutes.
Mr Strauss-Kahn's fall from power began when a New York hotel chambermaid accused him of sexual assault in 2011. Charges were dropped before the French allegations emerged and the maid and former French finance minister settled a civil suit out of court.
In court on Tuesday afternoon, he said that he played no part organizing the parties, but would just give others a date when he was available. "When I told them I was going to be in Paris, Fabrice [Paszkowski] would suggest either a lunch — or an afternoon, a bit more festive and sexually playful."
He said he had no idea that any of the women at the parties were prostitutes, and that the organizers hid this in order to make him happy. "It's true that many people wanted to make me happy," he said.
Mr Strauss-Kahn said he never had sex with prostitutes in part because it was too "dangerous" as they were liable to talk and hurt his "political ambitions". He added that sleeping with prostitutes was "not my idea of a sexual relationship" because "I like it to be a party".
The other defendants in court on Tuesday were a local police chief, the head of a medical supplies company and former executive at a construction company.
The trial began last week and is expected to last three weeks.