* First member of Spain's royal family to appear in court
* Lawyers ask for criminal charges to be dropped
* Pressure on legal system to get tough on corruption (Adds court to decide whether to drop charges before Feb. 9)
By Sonya Dowsett
PALMA DE MALLORCA, Spain, Jan 11 (Reuters) - Spain's Princess Cristina appeared in court on Monday on charges of tax fraud, the first member of the royal family to stand in the dock, as a lengthy investigation into her husband's business affairs finally went to trial.
King Felipe's 50-year-old sister is one of 18 people on trial after a six-year investigation into the Noos Foundation, a charity run by her husband Inaki Urdangarin, that prosecutors say was used to embezzle millions of euros in public funds.
Details of the probe badly damaged the image of the royal family at a time when ordinary Spaniards were grappling with a devastating economic crisis and tapped into popular disgust at cases of high-level corruption among bankers and politicians.
Cristina arrived with her husband at the Mallorcan court to be greeted by a barrage of TV and press cameras.
Dressed simply in a grey trouser suit, she sat motionless and unsmiling in the court room fronted by a framed photo of King Felipe during the nearly 13-hour hearing that finished at 1000 CET (2100 GMT).
Her lawyers asked the three female judges overseeing the trial to drop the criminal charges against her and the state prosecutor said there was insufficient evidence to back up the accusations.
The charges against the princess were privately filed by the 'Clean Hands' anti-corruption organisation using a Spanish legal instrument known as the 'people's accusation'. One of the judges said the court would decide before Feb. 9 whether to drop the charges.
The Spanish legal system is under pressure to get tough on corruption after a string of graft cases involving the establishment from the main political parties to bankers like former International Monetary Fund (IMF) head Rodrigo Rato.
"The law must be equal for all," said newspaper El Pais on Monday in an editorial which said the fact the probe had come to trial showed the legal system worked, albeit at a slow pace.
Urdangarin, a former Olympic handball player who was seated apart from his wife in the court, is accused of using his royal connections to win public contracts to put on events through the non-profit organisation.
Prosecutors say some of the foundation's money was transferred to a company largely owned by the princess and Urdangarin and used to pay for personal items ranging from parking tickets to children's birthday parties.
Cristina is charged with two counts of being an accessory to tax fraud and, if found guilty, could face up to four years in prison for each charge - a maximum of eight years.
Urdangarin is accused of nine crimes including fraud and tax evasion with a combined potential jail sentence of 19 and a half years. Urdangarin and his partners allegedly overcharged local governments for putting on conferences about sports and business, then hid the proceeds abroad.
The couple, who have four children, deny any wrongdoing.
The other 16 people in the dock included the former head of the Balearic Islands government and other high-ranking officials from the islands and the Mediterranean region of Valencia.
The case helped drive the Spanish royal family's approval rating to its lowest level since the monarchy was restored in 1975 after the death of dictator Francisco Franco in a historic move paving the way to democracy.
However, its popularity revived after former king Juan Carlos abdicated in 2014 in favour of his son Felipe, who is married to a former television journalist. The new king regularly tops approval polls of public figures.
Felipe was proclaimed king in a low-key ceremony and has worked to modernise the monarchy, paring down the royal family's formal members and taking duties away from his two sisters. He stripped Cristina of her title of Duchess of Palma in June.
After Monday's session, the court will adjourn and the trial will continue in February. The court hearing will end in June and sentences are expected to be handed out before the end of the year. (Editing by Janet Lawrence; Editing by Tom Heneghan)