Mevani was arrested by the Assam Police from Palanpur town in Gujarat on Wednesday night over a tweet and taken to Assam by air in the early hours, officials said.
Flaying the arrest of Mevani, Gandhi said it is an insult to the people who elected him as a public representative.
"Modi ji, you can try to crush dissent by abusing the state machinery. But you can never imprison the truth," Gandhi said on Twitter, using hashtag "#DaroMat" and "#SatyamevaJayate".
He tagged a report saying that Assam Police has taken Mevani into custody over his tweets on Prime Minister Modi.
"The undemocratic manner in which Assam Police has detained Gujarat MLA Jignesh Mevani, is anti constitutional move and an insult to the people who have elected him as a public representative. Congress workers will raise their voice against oppression," the Congress said on its official Twitter handle in Hindi.
Congress leader KC Venugopal also dubbed Mevani's arrest as unlawful and unconstitutional and alleged authoritarianism by the BJP.
"The unlawful and unconstitutional arrest of Jignesh Mevani at midnight by Assam Police is the latest proof of authoritarianism under BJP. Such an arrest of a people's representative not only betrays their fear of criticism but also attacks the foundations of our democracy," he said in a tweet.
According to a document shared by officials of the Assam Police, an FIR has been lodged over a tweet by Mevani few days back. However, the tweet has been withheld by Twitter. It was about Nathuram Godse.
Mevani is an independent MLA from Vadgam seat in Gujarat's Banaskantha. He recently joined the Congress.