Former Brazilian football star Ronaldinho and his brother have been detained in Paraguay after allegedly using fake passports to enter the South American country, authorities said Wednesday.

Paraguayan police raided the hotel in Asuncion where the 2005 Ballon d'Or winner was staying during a trip promote a book and found the false documents, the minister said.

Ronaldinho and his brother were not taken to the police station but remain under custody and are obliged to go to the prosecutor's office on Thursday.

"(Ronaldinho and his brother) are not being detained, they are still under judicial investigation and they have to testify. Then it will be decided whether they will be arrested or not," Paraguay's interior minister Euclides Acevedo said.

He further added that the pair believe that they were tricked by a man who was accompanying them, who has since been arrested and are cooperating in the investigation.

"Mr Ronaldinho has a false passport. That is a crime and that's why his arrest has been ordered," Paraguay's interior minister Euclides Acevedo said. "I respect his sports popularity, but the law needs to be respected no matter who you are."

Acevedo said that an investigation was "ongoing", adding that more information would be shared with the public at a press conference. "Ronaldinho will be heard at eight in the morning on Thursday at the prosecutor's office," Acevedo told reporters. "We are also investigating the responsibility of the authorities who allowed them to enter the country with falsified documents," Acevedo said.

The brothers arrived in Paraguay on Wednesday morning to participate in a children's charity campaign and to publicise a new book about Ronaldinho's career.

It is not the first passport controversy to engulf Ronaldinho and Assis. In November 2018, they had their travel documents seized by Brazilian authorities after failing to pay a fine resulting from a 2015 court case.

Ronaldinho won the UEFA Champions League with Barcelona in 2006 and before that he had won the FIFA World Cup in 2002.

Ronaldinho and his brother had their Brazilian passports withdrawn in November 2018 after failing to pay a $2.3 million fine for environmental damage incurred during the building of property in Rio Grande do Sul, in southern Brazil.