But Mr Khan is in a stable condition, his team say, and could potentially be discharged in the coming days.

The attack on Mr Khan has electrified the country, which the cricketer-turned-politician led until April, when he was ousted by a parliamentary vote of no confidence.

Following Thursday's shooting, his party - the Pakistan Tehreek-e-Insaf (PTI) - called for nationwide protests after Friday prayers. President Arif Alvi - a founding member of the party - called it a "heinous assassination attempt". Schools have been closed in the capital, Islamabad.

Mr Khan's political opponents have also been quick to condemn the attack, with Prime Minister Shehbaz Sharif ordering an immediate investigation.

Meanwhile, US Secretary of State Antony Blinken called for calm, saying: "Violence has no place in politics, and we call on all parties to refrain from violence, harassment and intimidation."

Pakistan - which is reeling from an ongoing economic crisis and devastating floods - has a record of political violence, with former Prime Minister Benazir Bhutto assassinated in 2007. Many evoked her killing in the wake of the attack on Mr Khan.

How the attack unfolded

Mr Khan - who has been fighting to return to office since he was ousted earlier this year - had been leading a "long march" of protests calling for early elections to facilitate his comeback.

By Thursday, his convoy had reached Wazirabad, where crowds had gathered to hear him speak.

He was stood on top of an open truck-bed surrounded by aides and his other party members when the shots rang out.

"It was so sudden that it took me a while to understand what was going on," one party staffer, Mueezuddin, told the BBC.

However, from their vantage point they were able to pick out the attacker.

"We saw the attacker had emptied a whole magazine," Mueezuddin said, "[and he had] loaded another magazine when he was grabbed by a boy from behind."

Unverified videos on social media show an assailant in the crowd pointing his pistol at Mr Khan's convoy before being overpowered by Mr Khan's supporters.

Footage of the incident and witness accounts suggested a security guard was also seen firing from the container.

Mueezuddin said Mr Khan and those around him ducked quickly after the first shots, and when he was hit he remained calm while he was given first aid by his bodyguards.

He was then moved into a bulletproof car and rushed away to hospital in Lahore.

Punjab Chief Minister Pervez Elahi has suggested there may have been more than one attacker, saying Mr Khan had been "shot in the leg from the front while the alleged attacker who was caught on the spot was on the right side".

Dispute over possible motive

Immediately after the shooting, some of his allies told media that Mr Khan believed the current political leaders - including Prime Minister Sharif, the interior minister and a military general - were behind the attack.

One spokesman, Raoof Hasan, told the BBC's Newshour programme the government was "attempting to eliminate [Imran Khan] physically".

Pakistan's leaders have not yet addressed such allegations.

Mr Khan, who remains popular, has previously characterised his removal as prime minister as a political conspiracy and been loudly critical for months of the current government and military leaders. Courts have convicted him in recent corruption cases but he has disputed the verdicts as politically motivated.

Police on Tuesday night released a video confession of the man who they said had attempted to kill Mr Khan.

It's unclear the conditions under which the interview was carried out. But, in response to police asking him why he had opened fire, he said: "He was misguiding the people. I wanted to kill him. I tried to kill him."

The video has been dismissed by Mr Khan's allies as a "cover-up".

One suspect remains in custody but no charges have been laid.