An Associated Press reporter at the scene saw between 20 and 30 people getting arrested at the rally.
A dozen workers from Khan's political party were arrested for attacking officers and blocking the road, police said.
Although Khan will not be on the ballot for the February 8 election, he remains a potent political force because of his grassroots following and anti-establishment rhetoric.
He says the legal cases against him were a plot to sideline him ahead of the vote.
Senior police superintendent Sajid Siddozai said workers from Khan's Pakistan Tehreek-e-Insaf (PTI) party organised the rally without obtaining permission from authorities and blocked the road.
Siddozai confirmed the use of tear gas.
"When police officials attempted to negotiate and persuade them not to block the road, they attacked the police," he said.
"This resulted in injuries to five police officials, including a female officer. One of the wounded is in a critical condition."
Police fired tear gas to disperse supporters of the Pakistan Tehreek-e-Insaf at an election rally. (AP PHOTO)
The police operation was ongoing, Siddozai added.
PTI worker Waheedullah Shah said Khan had called for rallies across the country and that Sunday's event in Karachi was peaceful.
"But police dispersed our rally and arrested our workers," Shah said.
"We will not be deterred by such tactics. We stand by Khan and will always support him."
There were violent demonstrations after Khan's May 2023 arrest.
Authorities have cracked down on his supporters and party since then.
Pakistan's independent human rights commission has said there is little chance of a free and fair parliamentary election next month because of "pre-poll rigging".
It also expressed concern about authorities rejecting the candidacies of Khan and senior figures from his party.
