Shops, markets and schools were closed in Karachi and demonstrators demanded the government seek out and arrest people involved in the attack on the Hazara community of Shiites, Geo TV News reported.
The blast Saturday injured at least 169 people, including women and children, Dawn News reported.
More than 20 men and women received life-threatening injuries.
About a dozen victims were so badly burned they could not be immediately identified.
The explosion, felt throughout Quetta, destroyed four markets and 100 shops.
A spokesman for the Shiite organization Majlis-e-Wahdat-e-Muslimeen said the sit-ins would continue "unless Quetta is handed over to the army for security," The News International reported.
The group's leader, Sajid Naqvi, blamed the attack on the lack of effective action against terrorists by the government.
"We have become a community of gravediggers," said Syed Dawood Agha, president of the Balochistan Shia Conference.
About 2,500 women from the Hazara community, a Persian-speaking people, conducted a demonstration protesting the carnage and later held a sit-in with coffins containing bodies of some of the victims of the attack.
A separate gathering on Sunday commemorated the deaths of 100 people in a Jan. 10 attack in Quetta.