Most shops, offices and schools in Karachi, a city of 18 million people, were closed and public transport reduced as the mourners gathered.
Kamal Hyder, Al Jazeera's correspondent in Islamabad, said: "Right now the people of Karachi are in mourning. Once they have buired their dead, there will be more anger that the security have not been able to protect them.
"Karachi has seen violence in the past. It would appear that whoever is conducting these acts of terror in Karachi wants to destabilise Karachi," he said.
"There has been a lot of talk that Karachi is a strategic city, it is a port city, but whoever wants to hit Karachi primarily wants to do so because it is the financial heart of the country.
Two bombs planted on buses carrying Shia worshippers were detonated simultaneously on Friday, before a third bomb blew up outside the hospital where the wounded were being taken.
Shia Muslims in Pakistan were marking Arbaeen - the end of a 40-day mourning period for Imam Hussein, a grandson of the Prophet Muhammad who was killed in a seventh century battle in Karbala.
Hasan Abdullah, a correspondent with Pakistan's Dawn News television, told Al Jazeera: "The police say that a bomb was planted on the first and second bus and they were detonated remotely."
Initial investigations had suspected a bomb-laden motorcycle was driven into one of the buses.
"At least 20 suspects have been rounded up, and police say these suspects belong to banned sectarian organisations," Abdullah said.
Raja Umer Khattab, a senior police investigator, said the Jundullah [Army of God] group was behind the attacks.
"This is the same group that carried out the Ashura attack," he said, referring to a bomb attack at a Shia procession in late December that killed 43 people.
National and local government officials urged calm following the blasts.
|Attacks on buses carrying Shia worshippers were followed by a blast at a hospital [AFP]
Shah Mehmood Qureshi, Pakistan's foreign minister, said he "condemned the Karachi bombings and said terrorism will never be allowed to succeed in its nefarious designs," according to a foreign ministry statement.
The statement quoted Qureshi as saying that "such acts only strengthen our resolve to fight terrorism".
No one has claimed reponsibility for the blasts, but the Pakistani Taliban has claimed past attacks against Shia Muslims in Pakistan.
Pakistan had tightened security in the city to protect mass processions of worshippers during Ashoura, deploying tens of thousands of police and paramilitary forces.
Karachi has a history of religious violence between Shia and Sunni Muslims, but in recent weeks clashes between rival political parties have left dozens dead.