Police said the 59-year-old, whose given name is Ulrich Schulz, was taken to the hospital for unspecified health issues after his arrival, though he appeared to be in relatively good health, chief detective Ignaz Raab said.
"The accused was emotionally agitated," Raab said. Investigators have said Shanti has chosen to remain silent regarding the criminal accusations.
"Police are looking for other victims," said state prosecutor's office spokesperson Anton Winkler in Munich. "The prosecutor's office wants to press charges quickly."
The Hamburg-born singer, who has been wanted since 2002, was arrested after being recognized at the German Embassy in Lisbon when he applied for a passport, which authorities believe he may have wanted to use to flee to Brazil, news agency AFP reported.
The authorities are not clear on how Shanti could have remained undetected in Portugal for so long. He was considered to be a model citizen of his village, Vila Nova de Cerveira.