In first incident, unidentified men hurled a grenade at a police van and opened fire on the busy M A Jinnah road, killing constables Mubeen and Shamsul Irfan on spot. Another constable was injured in the attack, they said.
In the second incident few minutes later, unknown gunmen opened fire on another police mobile in Gulshan-e-Iqbal area in which constable Matiur Rehman was killed.
Another constable was injured in the attack.
"We have now issued explicit instructions to all police stations not to allow patrolling by single police mobiles or motorcycles," DSP Munir Sheikh said.
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"We have strictly forbidden the practice of policemen patrolling on motorcycles as they are soft targets," Sheikh said.
In 2014, over 150 policemen on duty have been killed in targeted attacks.
On a day of violence two other civilians were also killed in firing incidents in other parts of the city.