"I strongly condemn the treatment meted out to Yogendraji by Delhi Police," Kejriwal said in a tweet.
"They were protesting peacefully. It is their fundamental right," the chief minister added.
Yadav claimed Delhi Police manhandled and beat him.
"I have been beaten up, manhandled, dragged, pushed and arrested at the parliament street police station," he tweeted early Tuesday along with a picture of himself.
Yadav was arrested while he was leading a protest by farmers from Punjab, Haryana and Rajasthan at Jantar Mantar against the government's proposal to change the way farm land is acquired for infrastructure projects.