Karachi Metropolitan Commissioner Matanat Ali Khan was hurt when someone threw a hand grenade at his car, Dawn newspaper reported. Two others, including his security guard, were injured.
About an hour later, an explosion occurred in an apartment in the Patel Para area, Dawn said. The neighborhood is close to the center of Karachi, which is also Pakistan's major seaport.
The International Herald Tribune, citing information from the Urdu-language Express News, said the blast was in an apartment on the top floor of the three-story building. The building, which was under construction, was demolished, leaving a one-foot crater.
Investigators said ball bearings were found at the site, suggesting the apartment was a bomb factory.
They also discovered cellphone batteries and SIM cards.
"No doubt, it was the terrorists' hideout but we have yet to ascertained which group it was," Raja Omer Khattab, head of the police counter-terrorism and financial crime unit, told the Express News. "We have also found the clothes of the women and children from the house but no woman and child was presented there at the time of blast."
The owner of the house said he had rented it to a man named Matinullah, who was recommended by a neighbor.
Three men were killed immediately. Matinullah died after being taken to a hospital, while a fifth man was taken into police custody at the hospital.