More than 70 people have died due to the violence in the last couple of days.
Media reports state that the police personnel were ambushed late night on Aug 19 in Chakra Goth area of Korangi, but two of the gunmen were captured after being wounded in the ensuing shootout.
City police chief Saud Mirza was quoted as saying, "The policemen were in plain clothes when they were ambushed and retaliated immediately."
The gunmen after attacking the van disembarked the policemen and shot dead four of them before the other commandos retaliated.
The police chief added, "Around 35 policemen, who were wounded in the ambush, have been shifted to Jinnah hospital the condition of six is critical." The injured included a DSP from Quaidabad.
The incident took place on a day when despite President Asif Zardari chairing a high level meeting in Islamabad to discuss the law and order situation in Karachi. The violence escalated with the death toll reaching 27.
Besides the killings, there were reports of 18 people missing and gunmen opening fire on public transport in some areas.
Seven vehicles were also torched in different parts of Karachi as the death toll for two days of violence has reached around 70.
(With PTI inputs)