The 70-year-old populist leader of the opposition Pakistan Tehreek-e-Insaf (PTI) party was hit by at least one bullet in his right leg, his senior aide, Rauf Hassan, confirmed to VOA.
The attack in Wazirabad town, central Punjab province, left at least one person dead and 14 others wounded, including the former prime minister.
Khan was transported to a hospital in Lahore, the capital of Punjab, about 150 kilometers from the site of the attack, where doctors said he was in stable condition, according to the aide.
Video footage showed a gunman firing from the ground at Khan, who was atop a purpose-built truck leading the protest march.
Witnesses said a participant quickly attempted to overpower the suspected shooter while he was still firing with his automatic weapon and fatally hit a marcher.
Police later took the suspected assailant into custody. In a purported video confession later released to reporters, the suspect said he acted alone and that his only mission was to kill Khan for "misleading" the public.
PTI leaders questioned the identity of the detained suspect, however, and his subsequent video confession, saying that it was an attempt to cover up the assassination plot and that there had been more than one assailant.
Provincial police later confirmed that there were two shooters and that efforts were under way to arrest the second person.
"It was a well-planned assassination attempt on Imran Khan, the assassin planned to kill Imran Khan and leadership of PTI," tweeted Chaudhry Fawad Hussain, a central leader of Khan's party. He said that "it was a burst from automatic weapons" and a "narrow escape" for his chief.
Faisal Javed, a close Khan associate and member of the Senate, the upper house of parliament, was among those injured. He also confirmed the casualties while speaking to reporters outside a local hospital in his bloodstained clothes.
Thursday's attack angered Khan's supporters, who took to the streets of major Pakistan cities to protest the attempt on his life.
Prime Minister Shehbaz Sharif condemned the shooting, saying he had directed the interior minister to immediately submit a report on the incident.
"I pray for the recovery and health of PTI chairman & other injured people. [The] Federal government will extend all support necessary to Punjab gov't for security & investigation. Violence should have no place in our country's politics," Sharif tweeted.
Khan issued a statement from his hospital bed, accusing the government and the military of plotting the attack, allegations officials denied.
The incident has been condemned by the U.S., U.N., Saudi Arabia and Canada among others.
Khan launched his so-called long march from Lahore last Friday, saying he and his supporters planned to stage a sit-in in Islamabad to press the Sharif government into holding early elections.
He was ousted in a vote of no confidence in April but rejected his removal as illegal, saying it was orchestrated by the United States in collusion with Sharif and Pakistan's powerful military — an allegation he has yet to substantiate with evidence and that Washington and Islamabad deny.