Zbigniew Huminski, 38, was charged with rape of a minor and kidnapping followed by death and remanded in custody on Friday.
Investigators said he confessed to the crime shortly after being arrested on Wednesday.
The autopsy revealed that nine-year-old Chloé had been sexually abused before being strangled and medical evidence confirmed Huminski's guilt, prosecutors said.
He kidnapped Chloé while he was heading for Britain via Calais after she squirted him with her water pistol, according to his lawyer, Antoine Deguines.
Some 5,000 people turned out for a march organised by the town council on Thursday evening and a further one was organised by individuals for Saturday.
The fact that Huminski is Polish and has already served two prison sentences in France - one for extortion with violence, the other for theft and sequestration - has sparked criticism from some right-wing politicians.
Laurent Wauquiez of the mainstream UMP on Friday tweeted that the case was a result of Justice Minister Christiane Taubira's penal reform, which has encouraged non-custodial sentences and shorter prison terms, and a "weak" Europe.
Marine Le Pen of the far-right Front National blamed UMP leader Nicolas Sarkozy, claiming that when in power he put an end to the deportation of foreign criminals, while her deputy, Florian Philippot, blamed government permissiveness and Europe's open borders.
Prime Minister Manuel Valls hit back, accusing the critics of exploiting "this abominable, horrible, appalling crime" for political purposes.
The Sarkozy-era change in fact did not abolish deportation but simply stopped it being automatic and Huminski had been ordered to leave France.
That order was later judged to lack a legal basis but he had been extradited to Poland to face trial for burglary.
He was sentenced to a year in prison on that charge but had not yet been locked up, leading the French authorities to complain that Poland had failed to stop him returning to France.
The Warsaw court expressed regret over the "immense tragedy" on Friday but insisted that it had followed procedure correctly.
It also pointed out that France had failed to inform it that a French psychiatrist had found that Huminski was a dangerous psychopath before his trial in 2010.