French gossip magazine Closer and regional newspaper La Provence in September 2012 published long-lens photos of the Duchess of Cambridge sunbathing topless in the south of France, triggering outrage in Britain.
The prosecution in the invasion of privacy trial called for "very significant fines" over the grainy pictures, while a lawyer for Closer said the royal couple wanted 1.5 million euros ($1.6 million) in damages.
The lawyer Paul-Albert Iweins insisted that the photos showed a "positive image of the couple" and criticised an "Anglo-Saxon reasoning of damages and interest".
The pair were snapped on a break at a chateau owned by Viscount David Linley, the son of Princess Margaret, the late sister of Britain's Queen Elizabeth II.
One of the most intimate shots, which were offered to some publications but not published in Britain, shows the Duchess of Cambridge topless.
Laurence Pieau, Closer's editor in France, Ernesto Mauri, chief executive of the Mondadori group which owns the magazine, and Cyril Moreau and Dominique Jacovides, two Paris-based agency photographers suspected of taking the pictures, face charges of invasion of privacy and complicity at the trial in Nanterre outside Paris.
La Provence's publishing director at the time, Marc Auburtin, and photographer Valerie Suau are also standing trial.
The case had been delayed for four months to give the lawyer for the agency photographers more time to prepare their defence.
Prince William's Kensington Palace office declined to comment ahead of the trial when contacted by AFP.
The case was being heard shortly after the royal couple visited Paris in March.
Police said two photographers were confirmed to be near the chateau at the time of the royal couple's stay after combing through lists of hotel guests and telephone numbers.
But both photographers have denied taking the photos, despite evidence that both received substantial payoff amounts after the images' publication.
The magazine has steadfastly refused to divulge the identity of the photographer who took the topless pictures.
A week before Closer published the shots, other images of the couple from a different angle were printed in La Provence.
La Provence photographer Suau has been charged for taking photos of Kate in a swimsuit in the same place, but her paper has denied that their photographer took any topless images.
The snaps triggered a furious reaction from the British royal family and a furore in Britain where several newspapers had rejected an offer to buy the pictures.
The angry royal couple launched legal proceedings soon after they were published.
Their lawyer argued that the photos were particularly distressing as it brought back painful memories of William's late mother Princess Diana's death in a car crash in Paris in 1997 while being pursued by paparazzi.
French authorities sided with the couple by banning any further reproduction of the pictures before launching a probe into how the snaps were obtained.
But the topless photos still appeared in several other European publications in Italy's Chi, which, like Closer, is owned by Mondadori, in Ireland's Daily Star and sister magazines in Sweden and Denmark.
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