Khan fell as the lift was just short of the 4.5 metre (15ft)-high platform in Lahore, witnesses said.
Television footage showed the dazed leader of Pakistan's Movement for Justice party being led away to hospital with blood on his head.
|Photo: Pakistani Politician Imran Khan Falls From Lift (stage) During rally in Lahore|
Dr Faisal Sultan, head of the Shaukat Khanum cancer hospital, said: "Imran has a head injury but he is stable, talking and recognising people. We will conduct some tests and further examinations."
Khan's party is not expected to win the election on 11 May but it could be a major partner in a coalition government.
He has promised to cut down on the tax evasion and corruption that have bled Pakistan's treasury dry.
Imaran Khan was part of the Pakistan team that won the cricket World Cup in 1992.
|Pakistani Politician Imran Khan Falls From Lift (stage) During Campaign-photo|