MADRID -- Spanish police on Friday arrested the leader of an anti-corruption group that brought criminal tax fraud charges against Princess Cristina, an official said.

Miguel Bernad, leader of Manos Limpias (Clean Hands), was detained by financial crimes officers at his Madrid home, according to a Spanish police official, who declined to provide more details. The police official spoke on condition of anonymity because of police policy.

The group pursued tax fraud charges punishable by up to eight years in prison against Cristina, the sister of King Felipe VI, after prosecutors declined to do so, saying Cristina should face an administrative fine at most.

Cristina's trial is underway in the city of Palma de Mallorca. Testimony continued Friday without her presence, which is not required at this stage during witness testimony.

Manos Limpias did not immediately reply to an emailed message seeking comment on Bernad's arrest. The Spanish police official had no information on whether Bernad had a lawyer.

The Manos Limpias lawyer questioning witnesses in the case, Virginia Negrete, told reporters in Palma de Mallorca she had no details about why Bernad, also a lawyer, was arrested.

The landmark trial started in January, with Cristina appearing in court and answering questions as the first member of Spain's royal family facing charges since the monarchy was restored in 1975.

Cristina is accused of failing to declare as income personal expenses paid by a real estate company she owned with her husband Inaki Urdangarin, an Olympic handball medallist -turned-businessman. Urdangarin in the same trial face charges of embezzling up to 6.2 million euros ($6.8 million) from contracts for sports conferences and events that were allegedly inflated or never performed.

The princess' husband, formerly the Duke of Palma, is accused of using his title to land the deals for the non-profit Noos Institute he ran with business partner Diego Torres.

Lawyers for Cristina, Urdangarin and Torres say they are innocent.