The daughter of Spanish King Juan Carlos is due to appear before a judge over fraud allegations in a corruption scandal.
Princess Cristina, 48, will answer tough questions on Saturday in court on the island of Majorca in the rare royal spectacle that has raised questions over the future of the monarchy.
Pro-republican campaign groups have vowed to demonstrate nearby.
The princess is accused of being complicit in the business dealings of her husband Inaki Urdangarin, who is also under investigation, but they have not been formally charged with any crime. Both of them deny wrongdoing.
Judge Jose Castro has spent more than two years investigating allegations that Urdangarian and a former business partner embezzled $8.18m in public funds via a charitable foundation.
Cristina was a member of the foundation's board and with her husband jointly owned another company, Aizoon, which investigators suspect served as a front for laundering embezzled money.
State prosecutors say there is no case to answer against Cristina, but the judge has allowed suits brought by pressure groups.
It is the first time a member of the royal family has gone to court as a suspect.
Juan Carlos won widespread respect for helping steer Spain to democracy after the death of the dictator Francisco Franco in 1975.
But the royals' popularity has plunged since the case against Urdangarin opened three years ago.
The king's woes were worsened by a luxury elephant-hunting trip he made to Africa in 2012 as his subjects suffered in a recession.
These scandals and the sight of the king looking frail and on crutches in his rare public appearances have raised debate about the future of his reign.
A recent poll showed 62 percent of Spaniards in favour of his abdication. Support for the monarchy in general fell to just under 50 percent.