Eighteen people have been charged over alleged efforts to abduct more than 100 children in Chad for passage to Europe.
The workers say they were assured that the children were orphans
Charges including kidnapping were laid against nine French aid workers and journalists, seven Spanish flight crew and two Chadian nationals.
The Europeans were detained in the city of Abeche on Thursday as they prepared to fly 103 children out of the country.
The charity behind the flight protested its innocence, saying it believed the children were orphans from Darfur.
However, staff from the UN children's agency Unicef say many of the children, now being kept in an orphanage in Abeche, cry at night for their parents and say they are from villages in Chad.
Earlier Chad's Interior Minister Ahmat Mahamat Bachir warned the aid workers from the French charity Zoe's Ark could spend several years in jail.
French President Nicolas Sarkozy has described the charity workers' actions as illegal and unacceptable, but a senior French opposition MP told the BBC Chad was trying to exploit the situation for political purposes.
"I know some of these people who are working in Darfur, and they are very generous people, and probably they have made several mistakes - it's not easy to understand," said Jack Lang, a former Socialist minister.
"And the French government in the first time was not against this operation, it was a humanitarian operation.
"But now it appears that the Chadian government wants to create with France difficult relations."
Spain's foreign ministry said it "disagreed" with the charges against its citizens.
"Everything seems to indicate there is no conclusive evidence against the Spaniards," Bernardino Leon, a senior ministry official, told Spanish radio.
"Until this evidence is presented, the government will continue to maintain that they are innocent and continue to demand their release."
The nine French detainees will face charges of attempted child abduction and fraud, the prosecutor's office said. The seven Spaniards will be charged with being accomplices to the crime, as will the two unidentified Chadian nationals.
It now looks likely the prisoners currently being held in Abeche will be transferred to jail, though whether they will be moved to the capital N'Djamena is unclear, reports the BBC's Stephanie Hancock.
Another European - a Belgian pilot - is in detention in N'Djamena, but is not reported to be facing charges at present.
Our correspondent says it could be some time before those charged appear in court to enter a plea.
She says the Chadian justice system moves notoriously slowly, but that faced with such a high-profile and sensational case the judiciary may well move more quickly than normal.
On the runway
Our correspondent was among a group of reporters taken by Chadian authorities to the airport at Abeche, a town close to the Sudanese border, and shown the private charter plane still sitting on the runway where it was abandoned three days ago.
Chad has shown photos of the jet said to be the one chartered
The reporters were also taken to the orphanage where the children are being cared for by aid workers and UN staff.
The majority of the children are believed to be between three and five years old, with the oldest about eight or nine, and several babies no more than one and a half, our correspondent says.
The charity insists it was trying in good faith to take endangered children abroad for medical treatment. But aid workers said they were not treating any of the children for any serious illnesses or injuries.
The reporters were also later taken to local police headquarters to see the 16 detained Europeans. They are not being held in prison cells but in a large room and are showing no signs of mistreatment, our correspondent says.