The bombs exploded at the entrance of the shrine to Sufi saint Abdullah Shah Ghazi as devotees packed it for a weekly gathering in Karachi's seaside Clifton district, police official Mohammad Nasim says.
"We are collecting details and we fear the casualties may rise," he said on Thursday.
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Provincial home minister Zulfikar Mirza said the dead included one woman and an investigation was already underway.
The shrine's floor was spattered in blood, said witnesses.
Slippers, sandals and flowers brought by devotees to lay at the tomb littered the area.
Witness Gul Mohammad said he was outside the shrine when two huge blasts were heard in quick succession.
"I rushed inside and saw blood and human flesh," he said.
"Some bodies were lying on the ground and several people wounded in the blasts were crying in pain. Then ambulances started arriving and moving the injured to hospitals."
Ambulances with sirens blaring were seen ferrying casualties to hospitals as police and paramilitary soldiers cordoned off the shrine.
There was no claim of responsibility for the latest attack but the Pakistani Taliban has been blamed for similar bombings in the past.
More than 3700 people have been killed in a series of suicide attacks and bomb explosions, many carried out by the Taliban and other al-Qaeda-linked Islamist extremists, in Pakistan during the last three years.
Prime Minister Syed Yusuf Raza Gilani "strongly condemned" the blasts in a statement and ordered an immediate inquiry into the incident.
"It is yet another heinous act of violence reflecting the deranged minds of terrorists," said Gilani, according to the statement issued by his office.
The latest attack came as the country struggles to deal with massive flooding that has killed nearly 1800 people and left millions reliant on aid handouts.
