Rt Revd Sadiq Daniel has also been appealing for people to pray for the country, following the attack which struck the outskirts of the south-western city of Quetta.
It happened at a polio vaccination centre and left 15 people dead, while 23 others were wounded.
Speaking to Premier Bishop Sadiq said: "People can't react... The only reaction is that they're scared. They're scared and they're very frightened.
"Our people, the nurses in the schools and the hospitals and all the volunteers will do all that they can do. And at the same time, we're praying.
"Just pray for peace. Not only for [the Diocese of] Karachi, but for the whole country.
"It was not expected. Forces are very vigilant; I don't think it will happen again. They will not allow any more."
Officials say 13 of those killed were police who were at the clinic to escort health workers, while a soldier and a civilian also died.
It understood health workers have been repeatedly targeted in recent years by Islamic militants and local police chief, Syed Imtiaz Shad, said vaccination teams were due to carry out an immunisation campaign in local neighbourhoods.
Provincial home minister, Sarfraz Bugti, said a suicide bomber detonated his explosive among the police officers.
Hours later, Ahmad Marwat, who said he was a spokesman for a little-known militant group called Jundullah, or Army of God, claimed responsibility for the blast, warning of future attacks on polio teams in future and not explaining why the centre in Quetta was targeted.
Islamic militants have targeted polio workers in Pakistan and their police escorts in recent years, accusing them of being spies for the United States.
Polio is endemic in Pakistan, however, some Pakistanis fear the vaccinations will sterilise their children.