A suicide bomber has smashed a truck packed with explosives into housing for a paramilitary force protecting Pakistan's largest city, killing at least one person.
Violence has escalated in recent years in the city as armed groups fight for control of land and resources, and militant groups like the Taliban have used the chaos to consolidate their foothold.
One body had been brought to the Abbasi Shaheed Hospital from the early morning explosion, Dr Saleem Memon said.
A truck driven by the suspected bomber smashed into the gates of a residential apartment block where members of the Rangers security forces live, said Javed Odho, deputy inspector general of the Karachi police.
The Rangers are a paramilitary force tasked with helping Karachi police maintain security. A spokesman for the force said at least 21 Rangers were wounded in the explosion.
Witnesses reported seeing a large plume of smoke in the sky. Pakistani television images of the blast sight showed what appeared to be an apartment block with a gaping hole in the middle where the bomb went off and part of the two-storey building was razed.
One of the Rangers, Muhammed Farooq, said he was preparing for work when he looked out the window and saw a vehicle smash through the main gate and into the building.
"Then there was a really big bang and I lost my balance and I saw a lot of smoke and then I lost consciousness," he said, speaking from the hospital.
There was no immediate claim of responsibility. Taliban militants are known to operate inside the city and have targeted security officials and buildings in the past.