The girl, named in reports as Rimsha Masih, has Down's Syndrome. She was accused by Muslims of burning pages of the Koran and was arrested by police after a mob surrounded her home in Mehrabadi, Islamabad.
Rimsha remains in police custody and hundreds of Christians have reportedly fled the area in fear. President Asif Zardari has called for an investigation into her arrest.
The protest in London is being organised by the British Pakistani Christian Association (BPCA).
BPCA chairman Wilson Chowdhry criticised Rimsha's arrest and the reported decision to try her in an adult court as opposed to a juvenile court.
"We believe President Zardari should be calling for her immediate release, especially considering her illness and tender age," he said.
"Any prolonged detention simply does not equate and time wasting could lead to irreparable damage to this vulnerable child."
Blasphemy carries the death sentence in Pakistan. Even if those accused of blasphemy are not sentenced to death, they face long prison sentences on top of the years they often have to spend behind bars awaiting trial.
There will be special performances from Catholic electro-pop group Ooberfuse, who are also the BPCA's protest band and have written music highlighting the plight of persecuted Christians in Pakistan.
Speakers at the protest include Michelle Huberman, creative director of Harif, a group serving Jews in the Middle East and the mother of a child with Down's Syndrome.
A representative from the charity Aid to the Church in Need, which has partners in Pakistan, will also address the crowds.