Pakistani officials say at least 13 people were killed and dozens wounded when a large explosion hit a protest rally near the Punjab provincial assembly in the city of Lahore on February 13.
The blast ripped through the crowd of hundreds of pharmacists, who were protesting new amendments to a law governing drug sales.
Mushtaq Sukhera, the inspector-general of police in Punjab, said five police officers were killed in what he described as a "suicide attack."
"The bomber exploded himself when successful negotiations were under way between police officials and the protesters," Sukhera told reporters.
Sameer Ahmad, the Lahore deputy commissioner, said 58 people were wounded, including nine who were in critical condition.
A loud bang could be heard on live television, which showed smoke billowing up as people ran from the scene, some of them carrying injured victims.
Jamaat-ur-Ahrar, a faction of the Pakistani Taliban, claimed responsibility for the attack.
The militant group warned that the attack was the start of a new campaign against government institutions.
The group also claimed responsibility for a bombing during Easter last year that killed more than 70 people in a public park.
In a statement, Prime Minister Nawaz Sharif said his country would continue to fight militancy.
"We have fought this fight against the terrorists among us, and will continue to fight it until we liberate our people of this cancer, and avenge those who have laid down their lives for us," he said in a statement.
Security in Pakistan has improved markedly in recent years but extremist groups such as the Pakistani Taliban and Islamic State still pose a threat and have carried out deadly attacks.