SSP Operations Zahid Gondal of Punjab Police and DIG Traffic Lahore Capt (retd) Ahmad Mobin were among those killed in the attack.
Hours earlier, DIG Mobin had been seen on television, mediating with the protesters gathered at the site.
Mobin was reportedly at the site to negotiate with protesters for an end to the sit-in and to clear the area.
A large group of chemists and pharmaceutical manufacturers were gathered in front of the provincial assembly to protest a government crackdown against the sale of illegal drugs. There was a significant presence of law enforcers in the area while the protest was ongoing.
Up to 400 people had attended the protest, according to an AFP reporter who was on the scene when the explosion occurred.