By Alexandra Alper

MEXICO CITY (Reuters) - The seven-year Mexican prison ordeal of a Frenchwoman convicted of kidnapping ended on Wednesday to cheers from supporters after the country's top court ruled her trial had been tainted and ordered her immediate release.

Florence Cassez, 38, was serving a 60-year sentence that opened up a diplomatic rift between France and Mexico after she was arrested in 2005 at a ranch near Mexico City with her former boyfriend, who led a kidnapping gang called the Zodiacs.

Supreme Court Judge Jorge Pardo ordered her immediate release after a nailbiting televised session, which at one point had looked to be going against her.

Cassez was still in prison, though her lawyers said they hoped she would be heading home to France on Wednesday evening.

"It's an explosion of joy. It's wonderful," Charlotte Cassez, her mother, told French television.

After the arrest, police made Cassez take part in a staged scene of officers freeing kidnap victims. She was portrayed as a kidnapper in the re-enacted event, which was aired on national television. Police subsequently admitted wrongdoing.

A judge sentenced her in 2008 following a closed-door trial with no jury, typical of most cases in Mexico.

Her lawyers had said Cassez's rights were violated and that evidence against her should be thrown out.

In March, Mexico's Supreme Court rejected a bid to release Cassez immediately, but opened the door to a review on Wednesday, which had initially been intended to discuss a motion to throw out some of the evidence used to convict her.

Critics of Mexican justice saw the Cassez case as a test of the system's ability to rectify its faults. However, the prospect of her release has stirred resentment among kidnapping victims. Thousands of serious crimes have gone unpunished by Mexico's justice system.

Former French President Nicolas Sarkozy backed Cassez's fight to be freed and her parents had said his successor, Francois Hollande, assured them he would work for her release.

(Additional reporting by Pauline Mevel and John Irish in Paris, and Gabriel Stargardter in Mexico City; Editing by Dave Graham, Simon Gardner and Mohammad Zargham)