The Muslim-majority nation's strict anti-blasphemy laws make defaming Islam or desecrating the Qur'an illegal and potentially punishable by death.
Rimsha, who is between 10 and 13 years old and is reported to have Down's Syndrome, was taken into custody in a low-income area of Islamabad on Thursday after furious Muslims demanded she be punished.
Polly Truscott, Amnesty International's South Asia director, said the case showed the "erosion of the rule of law" in Pakistan and the dangers faced by those accused of blasphemy.
"Amnesty International is extremely concerned for Rimsha's safety. In the recent past individuals accused of blasphemy have been killed by members of the public," Truscott said on Tuesday.
President Asif Ali Zardari on Monday ordered officials to explain the arrest, while Christians fled the neighbourhood of Mehrabad in fear at Muslim anger over the incident.
Truscott welcomed Zardari's response but warned it would count for little unless there were "greater efforts to reform the blasphemy laws to ensure they cannot be used maliciously to settle disputes or enable private citizens to take matters into their own hands."
'Failure to reform'
There has been growing concern in the West over religious intolerance in Pakistan following the assassinations last year of a leading politician and a Christian Cabinet minister who spoke out against the blasphemy law.
"The continued failure to reform these laws has effectively sent the message that anyone can commit outrageous abuses and attempt to excuse them as defence of religious sentiments," Truscott said.
Neighbours said Rimsha had burned papers collected from a garbage pile for cooking in her family home and someone alerted the local cleric after spotting the remains being thrown out as rubbish.
A Christian mother sentenced to death for blasphemy in late 2010 remains in prison, while last month, a mob snatched a mentally unstable man from a village police station and beat him to death in central Punjab province after he allegedly burned pages from a Qur'an. – AFP