The actress, who plays scientist Clarissa Mullery in the BBC crime drama, was rushed to hospital with a head wound following the attack near Euston station.
Liz, who suffers from Arthrogryposis Multiplex Congenita and is a wheelchair user, was with her carer at the time.
They were also injured trying to protect her, suffering a broken finger and minor cuts. Both have since been discharged.
A man in his twenties has been arrested and sectioned under the Mental Health Act.
Liz has since taken to Twitter to thank fans and friends for their well wishes, tweeting: “Thank you for all the love today and always xxx.”
In a statement, the Metropolitan Police said: “Police were called at 20:21hrs on Thursday, 10 August, to Euston Road, near to Euston station, to a reports of man attacking two women with scissors.
“Officers and LAS attended the location. The two women were taken to hospital suffering minor cuts and subsequently discharged.
“One of the victims was in a wheelchair. She was with her carer who was the second victim.
“A man – aged in his 20s – was arrested at the scene on suspicion of GBH. He was taken into custody at a north London police station. He was subsequently detained under the Mental Health Act. Enquiries continue.”