Television footage showed Khan, leader of the Pakistan Movement for Justice party (PTI), bleeding from the head as he was carried by aides through the crowd at the event in Pakistan's second largest city Lahore.
The dramatic development came at the end of a day that saw 17 people killed and dozens more wounded in bomb attacks in northwest Pakistan, taking the death toll in the bloody campaign for Saturday's general election past 100.
The poll will mark a democratic milestone in a country ruled for half its history by the military as the first time a civilian government has served a full term and handed over to another through the ballot box.
Khan, who won only one seat in 2002 and boycotted polls in 2008, has led an electric campaign, galvanising the middle class and young people in what he has called a "tsunami" of support that will propel him into office.
The 63-year-old, who has undertaken a punishing schedule of daily rallies, tumbled from a riser along with several of his staff, seemingly after one of them lost their balance.
"Imran Khan fell from a lifter. He has received injuries to his head and he has been taken to hospital," Malik Ishtiaq, a local spokesman for Khan's Pakistan Tehreek-e-Insaf (PTI) told AFP in tears.
Another party spokesman said Khan's injuries were minor.
"Imran Khan is all right. He has been taken to the hospital for first aid. He will be back to address the rally very soon after getting initial treatment," Chaudhry Rizwan told AFP by telephone.
PTI information secretary Shirin Mazari told AFP Khan had fallen from a height of more than seven feet (over two metres).
"He had an injury to his forehead and he is conscious. He is being taken to Shaukat Khanum Hospital," she said, referring to the cancer hospital that Khan set up in honour of his mother.
Hundreds of well wishers and party supporters gathered outside the hospital, chanting "Long Live Imran Khan", an AFP reporter said.
Tuesday's attacks took place in the northwestern town of Hangu, a flashpoint for violence between Sunni and Shiite Muslims, and in the northwestern district of Dir, where Pakistani troops crushed a Taliban-led insurgency in 2009.
The Pakistani Taliban has condemned the polls as un-Islamic and directly threatened the main parties in the outgoing ruling coalition led by the Pakistan People's Party (PPP) and other secular allies.
Twelve people were killed and more than 40 injured at Hangu when a suicide bomber targeted election candidate Syed Janan, said Musarrat Qadeem, information minister of Khyber Pakhtunkhwa province.
Janan, who is seeking re-election to the Khyber Pakhtunkhwa provincial assembly for the right-wing Jamiat Ulema-e-Islam, told AFP he had been wounded on his head and shoulder.
"I was on my election campaign and coming to my vehicle when the bomber blew himself up. I received some injuries but survived. Two of my guards were seriously wounded," Janan said.
Later on Tuesday a roadside bombing killed five people, including the brother of a provincial assembly candidate for the PPP who had gone door to door to canvass for votes in Dir, police said. Seven other people were wounded.
Taliban spokesman Ehsanullah Ehsan claimed responsibility for the attack in Dir, but denied involvement for the suicide attack in Hangu.
"Our attacks on the PPP, the ANP (Awami National Party) and the MQM (Muttahida Qaumi Movement) will continue," Ehsan told AFP in a telephone call from an unknown location.
The number of people to have died in attacks on politicians and political parties since April 11 has now risen to 109, according to an AFP tally.
On Monday, 23 people were killed at a rally in the tribal district of Kurram, the deadliest single attack on the campaign so far.
Elections have been postponed in three constituencies where candidates have been killed. They are in the southwestern province of Baluchistan, in Pakistan's biggest city of Karachi and in the southern city of Hyderabad.
The national campaign race has been dominated by the centre-right -- Khan and opposition leader Nawaz Sharif, head of the Pakistan Muslim League-N.