UAE princesses accused of servant abuses
Sheikha Hamda al-Nahyan and her seven daughters hired a floor of rooms at a luxury hotel over eight months in 2008.
They brought with them from the United Arab Emirates a retinue of more than 20 servants whom they are accused of holding in conditions close to slavery.
The plaintiffs say they were prevented from leaving the hotel and forced to eat the princesses' leftovers.
The princesses are being tried in absentia along with an Indian butler.
If found guilty, they could face hundreds of thousands of euros in damages and even a prison sentence - but rights activists say it is highly unlikely that the UAE would extradite them to serve time behind bars.
Nonetheless, it would be "hugely significant" if one of the wealthiest families in the world was publicly linked with trafficking and slavery, says Nicholas McGeehan, an expert on migrant workers in the Gulf for Human Rights Watch.
He argues that despite being abolished in law, domestic slavery continues in Gulf states - "perpetuated by ruling elites for whom it serves an important societal purpose in conferring status".
He added: "It's top-down and tolerated."
The princesses deny the charges, and the BBC has contacted their defence for comment.
The case came to court on Thursday and was hearing from defence lawyers on Friday morning.
One of the alleged victims told Belgian television that the women were detained in hotel rooms with private guards and prevented from going outside.
They had to be available to take orders 24 hours a day, slept on the floor in the princesses' rooms and forced to eat the princesses' leftovers. One who complained is alleged to have been deprived of food and water for three days.
As well as charges of inhumane treatment, the princesses are also accused of failing to procure the correct visas and work permits for their servants as well as failing to pay wages.
Legal challenges to proceedings by the defence have meant the case has taken nine years to get under way. They have challenged, for instance, whether the police had a legal mandate to enter the princesses' hotel suites.
The case was cleared to go to court in 2010, but the case has been stalled by procedural challenges, Stef Janssens told the BBC. He works for Belgian rights organisation Myria which has been supporting the alleged victims.
"The princesses are of course very important people with immense means and prestige, while the victims are very vulnerable," he said.
"The princesses hired three specialist lawyers who twice went to Belgium's highest court to challenge procedure. Not everyone has the means to do that."