The cadres of the Chhatra League, the student wing of the ruling Awami League, attacked Mahmudur Rahman, 66, with bricks and bamboo sticks, his colleague Mohammad Abdullah said.
The attack happened when he got into a car after being bailed over a defamation case by a court in the western town of Kushtia.
"It was a brutal attack. [They] hit the right side of his head with a brick. We are now heading to Jessore city on an ambulance," Abdullah said.
Images on social media showed Rahman bloody but conscious and able to walk.
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Rahman was the editor of the mass-circulation pro-opposition Bengali daily Amar Desh, which was shut down by the government in 2013. He also heads a pro-opposition professional group.
Abdullah said Rahman had been confined to the courtroom for five hours after more than 100 of the activists took up position outside.
"Some 60-70 people took part in the attack," he said, accusing police of inaction.
A police officer said they were busy providing security for a religious event.
Images of a bloodied Rahman went viral on social media.
Police confirmed the attack on Rahman was by ruling party student activists.
"After he got bail, Chhatra League activists attacked his car with brick pieces. He was hit by broken glass of the car," police inspector Muniruzzaman said.
Rahman spent more than three years in jail on periods of arbitrary detention and was released in November 2016.
He faces some 80 charges including sedition, torching of vehicles, a plot to kill the son of Prime Minister Sheikh Hasina and inciting religious tension.
According to rights group Reporters without Borders, media self-censorship is growing in Bangladesh as a result of the "endemic violence" against journalists and media outlets, and the "almost systematic impunity" enjoyed by those responsible.
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