Dawood Badani, a member of the outlawed Sunni Muslim Lashkar-e-Jhangvi group, was arrested in the port city of Karachi at the weekend, Major-General Javed Zia, chief of the rangers in Karachi, told reporters on Monday.
"He is a key member of the group," Zia said.
He was wanted for masterminding most attacks on Shia Muslims in Quetta, the capital of Baluchistan province, including an assault on a Shia procession which killed more than 40 people in March, Zia said.
"He was also involved in an attack on a Shia Muslim mosque in July last year that killed more than 57 people."
Badani fled to Karachi to escape arrest, he added.