A supporter was killed and 13 others were injured in what his party called an "assassination attempt" on the former cricketer.

The attack happened during his ongoing march to Islamabad at Gujranwala district, about 200 km from the capital, against the army establishment-backed central government.

With his right leg bandaged, Khan, 70, was seen waving as he was shifted into an SUV. He was later reported to be "out of danger".

The attack brought back chilling memories of the assassination of another former Pakistan PM Benazir Bhutto during a rally in 2007.

Meanwhile, in a video shared by the police later, the attacker, who was overpowered and arrested, said he wanted to kill Khan and was not backed by anyone.

"I wanted to kill Imran Khan as he's been misleading the people," he said on camera.

There were unconfirmed reports of another gunman, with an automatic rifle, at the spot.

The attacker, a young man, fired shots from below at Imran Khan as he stood atop a container truck.

At least four of Imran Khan's party leaders were among those injured, one of whom, parliamentarian Faisal Javed Khan said a supporter was "martyred".

Imran Khan was taken to Lahore's Shaukat Khanam Hospital, 100 km away, which he'd got built in his mother's memory in the 1990s.

What Pakistan Prime Minister said?

Prime Minister Shehbaz Sharif condemned the firing and directed Interior Minister Rana Sanaullah to seek an immediate report from the police and administration.

Foreign Minister Bilawal Bhutto Zardari of the PPP, whose mother Benazir was killed in similar firing, said: "I strongly condemn the attack on Imran Khan. I am praying for his swift recovery."

However. Imran Khan's party, Pakistan Tehreek-e-Insaaf (PTI), has raised fingers at the central government over the firing incident and demanded answers from Punjab's provincial government, which is run by another party.

In fact, barely an hour before the firing, Imran Khan had told supporters in another part of Wazirabad town, where he was scheduled to deliver a speech, that they should accompany him to a different area instead, and promised to speak there, Geo reported.

His party tweeted a video showing him boarding the container truck from his black SUV.

The firing took place minutes later as he got to the roof for his speech.