"It's a trial that exposes how men — in politics, in the business world, finance world, in journalism as well — how they can use their position of power," Mediapart editorial director Edwy Plenel said.
Baupin, a prominent Green Party politician and former Paris city official at the time, denied wrongdoing.
He sued the six women named in the reports, the journalists and two men who were quoted saying they witnessed some of the alleged misbehaviour.
Most of the women who spoke about were fellow Green Party members. Four filed criminal complaints for sexual harassment.
A nine-month judicial investigation ended without charges, with prosecutors saying the statute of limitations had expired.
Baupin was not in court today and doesn't plan to attend the trial. Defence lawyers requested an order forcing him to appear, but the judge said that as the plaintiff Baupin had the right not to be there.
The women accused of defamation said they resented the situation.
He brought on an accusation against us, but he is not here. So that's where my anger is.
It was "a disgrace" to be "sitting on the bench of the accused when this man hasn't even been judged," Elen Debost, an elected official in the town of Le Mans, in central France, said
Debost alleged she received dozens of unwanted text messages of a sexual nature from Baupin in 2011.
"We all said what he had to say (something) to protect other women. So that what happened to us doesn't happen to others," she said.
Annie Lahmer, a Green member serving on the Paris region council, alleges Baupin told her she would not be able to have a career in the party after he chased her around an office desk in 1999 and she made it clear his behaviour was unacceptable.
"It's complicated but at the same time, it was a necessary stage to go through," Lahmer said of deciding to go public with her allegations in 2016.
"It's even more absurd because Baupin is not here. He brought on an accusation against us, but he is not here. So that's where my anger is."
The trial is scheduled to continue tomorrow, and hearings also are scheduled on Friday and Saturday NZT.
In France, defamation is punishable by a maximum fine of 45,000 euros.
- AP