"We have arrested 40 suspects during a massive crackdown early Thursday in the outskirts of Karachi in connection with Ashura bombing," said Mohammad Waseem, a senior police official.
He added that officials would interrogate the detainees, adding that their identities would soon be disclosed.
At least 80 people were also injured in the Sunday attack on a procession of Shia Muslims mourning the martyrdom of Imam Hussein (PBUH), a grandson of the Prophet Mohammad (PBUH).
Pakistani militants based in South Waziristan claimed responsibility for the bombing.
Pakistan has experienced a wave of violence over the past two years, in which nearly 3,000 people have been killed in bomb attacks and other terrorist operations across the country.
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