Cristina, a 50-year-old mother-of-four with a master's degree from New York University, is the first Spanish royal to face criminal charges since the monarchy was reinstated following the 1975 death of dictator General Francisco Franco.
The princess and her husband, former Olympic handball medallist Iñaki Urdangarin, arrived together at a makeshift courtroom in Palma on the Mediterranean island of Mallorca as photographers snapped pictures and a police helicopter flew overhead.
Following courtroom rules, they had to sit apart as judges read out the alleged crimes committed by the total of 18 suspects in the case, which alleges that Urdangarin embezzled public funds through a foundation he once chaired.
Cristina has been charged with tax evasion while her husband is accused of the more serious crimes of embezzlement, influence peddling, document falsification, money laundering, forgery, breach of official duty and tax fraud.
No 'privilege' for princess
Almost immediately after the trial opened, Cristina's lawyers called for the case against her to be thrown out.
Prosecutors have always refused to press charges against her, but under Spanish law, private entities can also file criminal complaints – and that is just what anti-graft campaigners “Manos Limpias”, or “Clean Hands” did.
Cristina's lawyers cited Spanish jurisprudence which allows an accused to escape trial if the victim of a crime does not back the charges – and in this case the alleged victim is the state.
But Virginia Lopez Negrete, the lawyer representing “Manos Limpias”, rejected the argument.
“All citizens are equal before the law and as a result anachronistic doctrines cannot be applied” that would “privilege” the princess, she said.
Journalists from around the world have flocked to cover the trial, which was moved from a courthouse to a public administration school on the outskirts of Palma to accomodate the large number of reporters and lawyers.
It comes as Spain seethes over repeated corruption scandals that have exposed politicians, trade unions, bankers and footballers, eroding Spaniards' faith in their institutions and elites after a major economic crisis and a government austerity drive.
“We have never had as much corruption in Spain's democratic history,” said 45-year-old unemployed masseur Francisco Solana, one of a handful of protesters who gathered outside the courtroom.
“No judge will dare send Princess Cristina to jail. I think justice is not equal for all, it favours the rich,” added Solana who was wrapped in the yellow, red and purple flag of Spain's 1931-1939 second republic.
The case is centred on business dealings by the Noos Institute, a charitable organisation based in Palma which Urdangarin founded and chaired from 2004 to 2006.
The 47-year-old and his former business partner Diego Torres are suspected of embezzling €6.2 million ($6.7 million) in public funds paid by two regional governments to the organisation to stage sporting and other types of events.
Urdangarin is accused of using his royal connections to secure inflated contracts without competing bids and siphoning off some of the money into Aizoon, a firm he jointly ran with his wife Cristina to fund a lavish lifestyle.
The couple on their wedding day in April 1997. Photo: AFP
The couple are suspected of using Aizoon for personal expenses including work on the couple's mansion in Barcelona, dance lessons and even Harry Potter books, which reduced the firm's taxable profits, according to court filings.
If convicted Cristina – who has denied knowledge of her husband's activities – faces a jail term of up to eight years. Urdangarin faces more than 19 years in prison.
Did royal palace 'cooperate'?
The corruption scandal and health woes prompted Cristina's father Juan Carlos to abdicate in 2014 in favour of his son Felipe to try to revive the scandal-hit monarchy.
King Felipe VI swiftly ordered palace accounts to be subject to an external audit and promised an honest and transparent monarchy.
Torres, Urdangarin's former business partner, has insisted that Juan Carlos and his advisers knew and approved of his son-in-law's business dealings at the Noos Institute and has hundreds of emails that can prove it.
“The royal palace was informed, supervised, and at times even cooperated,” he said during an interview broadcast on private television La Sexta on Sunday.