France's foreign minister reacted strongly, calling the decision deplorable and saying it would affect relations with Mexico.
Mexico's Foreign Relations Department rejected the charge that an injustice was done and said it regretted "profoundly" that France would allow relations to be harmed by the case.
The court said in a statement that the conviction and 60-year-sentence of Florence Cassez would stand. The court said that prosecutors had proved Cassez's guilt in three 2005 kidnappings and that irregularities alleged by her defense attorney did not hinder the case.
Cassez has acknowledged she lived at a ranch near Mexico City where the kidnap victims were held, including an 8-year-old girl. But she said she was simply dating a Mexican arrested in the case and did not know the people at the ranch had been kidnapped.
One of the victims identified Cassez as one of her captors, and another suspect in the case said the Frenchwoman not only participated in abductions, but helped lead the gang that carried them out.
The appeals court ruled that while the victims never saw Cassez's face, they identified her by her voice, foreign accent and hair color.
Cassez's imprisonment became a hotly debated issue in France after Mexican police acknowledged they staged a televised raid of the ranch in which officers appeared to rescue the hostages and detain Cassez and a Mexican man. The Attorney General's Office acknowledged that, in fact, Cassez had been arrested the day before outside the ranch.
French Foreign Minister Michelle Alliot-Marie harshly criticized the court's decision, saying she was dismayed by the ruling and warning it "will weigh on our bilateral relations" with Mexico.
"This decision is deplorable," Alliot-Marie added in a statement.
"None of the fundamental legal or factual elements raised by Florence Cassez's defense were taken into account, as they should have been in a state of law," she said.
Alliot-Marie said the French government will not abandon Cassez and will explore all legal paths available.
Hours later, Mexico's Foreign Relations Department issued a statement saying it "rejects the assertion that justice has been denied" to Cassez. It said Cassez has always been represented by several lawyers and has had consular assistance from the French Embassy.
Defense lawyer Agustin Acosta argued that parading Cassez in front of the cameras after her arrest prejudiced the case from the beginning. But the appeals court rejected that argument, saying those videos were not formally considered during the trial.
"She was presented as guilty in front of the television," Acosta said. "We all know the impact of television and how it can influence judges and everybody."
Acosta also said the victims changed their testimony several times during the trial. He said that at first, three victims said they could not identify Cassez. But after the staging of the raid was discovered - two months after authorities first announced her arrest - two of the victims told police they could identify her, Acosta said.
He also said the victims described the events as they unfolded before the cameras during the staged raid - and not how they really happened. One woman described being rescued by uniformed and masked police and said she heard people climbing the walls and breaking down doors.
However, Acosta said police later acknowledged they were dressed in civilian clothing and were let into the ranch by Israel Vallarta, Cassez's ex-boyfriend who was also arrested.
One of the victims, Ezequiel Elizalde, said during a 2009 radio interview that Cassez threatened to mutilate and kill him, telling him to choose "a finger or an ear." He said he had a scar on his finger where Florence pricked it with a needle and said she was giving him anesthesia.
Acosta said Elizalde and police gave three different version of the day he was stuck with the needle. He said the defense presented forensic evidence that it was a birthmark - not a scar - on his finger and that the prosecutors never conducted their own forensic tests or disputed the defense's findings.
Acosta said Cassez might be able to appeal to Mexico's Supreme Court if lawyers can find evidence that the Mexican Constitution was violated.
He said her family would definitely take the case before the Inter-American Commission of Human Rights, a branch of the Organization of American States. The commission decides whether to take cases to the Costa Rica-based Inter-American Court of Human Rights, a process that typically takes years.
"I continue to be convinced of Florence's innocence," Acosta said. "The family is going to continue this battle to the end."
Cassez was initially convicted of four kidnappings and sentenced to 96 years, but in 2009 a court reduced the charges to three kidnappings and cut the sentence to 60 years.
She is the only person convicted in the case. The case against her former boyfriend, Vallarta, is pending.
Cassez has found little sympathy in Mexico, which has one of the world's highest kidnapping rates.
Two prominent anti-crime activists, Isabel Wallace and Alejandro Marti, had urged the court to keep her in prison. Their own children were kidnapped and murdered in unrelated cases.
"It's infuriating that someone who hurt Mexican society, like Florence Cassez Crepin, is trying to make herself a victim," the activists said in a statement signed Wednesday by the groups they lead and several other anti-crime associations. "The real victims were those who were kidnapped by Cassez's band."
The groups exhorted the judicial system and the Mexican government "respectfully but energetically, not to cede before the pressure of the French government."
The French Embassy complained the groups put inappropriate pressure on the court.
"It's worrisome to see that these organizations are exerting direct pressure over the judicial system to influence the decisions," an embassy statement said. "They are practicing what they denounce and what they are accusing - without cause - the French government of doing."
French President Nicolas Sarkozy visited Mexico in 2009 and asked Mexico to allow Cassez to serve her sentence in France. But President Felipe Calderon eventually rejected the request because France could not guarantee that Cassez would serve her full 60-year sentence.
Calderon has faced huge street protests over Mexico's rate of kidnappings and other crimes. There were 1,163 reported kidnappings in the country of 112 million people in 2009, the most recent year for which the government released statistics.
That rate - 1.1 per 100,000 inhabitants - is considered high for a crime that is rare in many other countries. And the government acknowledges that the real number is probably much higher because the vast majority of kidnappings go unreported due to mistrust of police.
Associated Press writer Alexandra Olson contributed to this report.