The suspect, identified as Muhammad Ilyas, was held from the Nasirabad area of Balochistan's Loralai district, some 100 km east of capital Quetta, Geo TV said.
The cleric, Sarfraz Ahmad Naeemi, was in his office adjacent to a mosque here when a teenage suicide bomber entered the building and blew himself up Friday.
Three others were also killed in the blast, which the Taliban has taken responsibility for.
Naeemi, a moderate cleric, was a staunch critic of the Taliban and played a major role in turning public opinion in Pakistan against extremism and terrorism.
He was a vocal supporter of ongoing military operations against the Taliban in Swat and three neighbouring districts. He repeatedly issued fatwas to declare suicide attacks were against Islamic principles.
The mosque blast, as also another explosion in the city Friday that too claimed four lives, came a day after security forces repulsed a militant raid on the residence of a regional army commander in the northwestern region, killing two attackers and arresting five after a shoot-out.
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