Police arrested Shesh Narayan Jha, who works for a Nepali magazine, on Monday together with a protester who splashed red paint on the walls of the Singha Durbar government complex.
Jha says he was simply taking photos but police say he was involved in the protest against alleged excessive use of force by security officials during recent anti-charter demonstrations, in which 50 people died.
The two men were released on bail late yesterday after Harijan filed the petition.
Nepal's chief media organisation condemned Jha's arrest Wednesday as a "serious violation of press freedom", amid fears over threats to freedom of expression in the country.
More From This Section
Last week a British tourist was arrested for allegedly joining a protest against the constitution. He was later released.
Nepal's new constitution, adopted last September, was meant to cement peace and bolster the nation's transformation to a democratic republic after decades of political instability and a 10-year Maoist insurgency which ended in 2006.
But the country's first charter to be drawn up by elected representatives sparked months of protests from the country's Madhesi ethnic minority, who said it left them politically marginalised.
Several rounds of talks between the government and the protesting parties have failed to secure an agreement.