Pakistani politician and former cricketer Imran Khan injured his head in a fall at an election rally Tuesday as the death toll in the bloody campaign topped 100 following fresh attacks.

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 in Pakistan's second largest city of Lahore before being taken to hospital.

The dramatic development came at the end of a day that saw 18 people killed and dozens wounded in bomb attacks in northwest Pakistan ahead of Saturday's general election.

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 60-year-old, who has undertaken a punishing schedule of daily rallies but who is known for his physical fitness, tumbled from a makeshift lift along with several of his staff, seemingly after one of them lost their balance.

Witness Raza Zaidi told AFP that Khan was being lifted on a crane with five people but lost his balance and fell when a sixth one tried to climb up. Khan hit the lifter before falling on the ground.

"Imran Khan has one head injury. We are conducting his CT scan and other tests. He is stable, he is conscious and he is recognising people. He is all right," Faisal Sultan, the director of Shaukat Khanum hospital, told reporters.

In a televised statement from his hospital bed Imran Khan urged people to vote for his party.

"I did whatever I could for this country. Now remember 11th May, come out and vote for PTI without considering its candidates, just vote for PTI," he said.

Supporters at the rally venue, where thousands had gathered, expressed their concern for Khan.

"Passion and love for Imran Khan brought all these people here. We are still here, all we can do is pray now," said Sobia Khan, a PTI supporter.

PTI spokeswoman Shirin Mazari told AFP Khan was injured on his forehead after falling from a height of more than seven feet (two metres), but was conscious.

Party officials had initially said he would be back to address the rally but he was later transferred from a small private hospital to the Shaukat Khanum cancer hospital he set up in honour of his mother for specialist treatment.

Television images showed Khan being transported on a stretcher, his head swathed in bandages.

Hundreds of well-wishers and party supporters gathered outside the hospital, chanting "Long Live Imran Khan", an AFP reporter said.

Khan's main rival, former prime minister Nawaz Sharif, tipped to win the election, conveyed his sympathies, his Pakistan Muslim League-N party spokesman Siddiqul Farooq told AFP.

Later, Sharif announced the cancellation of campaign activities on Wednesday.

Following fresh bombings on Tuesday, the number of deaths in attacks on politicians and political parties since April 11 has now risen to 110, according to an AFP tally.

The Pakistani Taliban have condemned the polls as un-Islamic and directly threatened the main parties in the outgoing ruling coalition led by the secular Pakistan People's Party (PPP).

Twelve people were killed and more than 40 injured in the northwestern town of Hangu when a suicide bomber targeted an election candidate, said Musarrat Qadeem, information minister of Khyber Pakhtunkhwa province. The candidate was wounded.

Elsewhere, a roadside bombing killed five people, including the brother of a provincial assembly candidate who had gone door to door to canvass for votes, in northwestern district of Dir, police said.

Another roadside bomb hit the car of a provincial assembly candidate in Upper Dir, killing his son, a local police officer said. The candidate was wounded.

Taliban spokesman Ehsanullah Ehsan claimed responsibility for the first attack in Dir, but denied involvement in the suicide attack in Hangu. He said attacks would continue.

Nobody immediately claimed the attack in Upper Dir.