A powerful bomb blast on Monday ripped through a protest in the Pakistani city of Lahore, killing at least 13 people and wounding dozens, according to officials.

The explosion went off in Lahore's busy Mall Road during a rally attended by hundreds of pharmacists protesting against changes to a drug sale law outside the provincial assembly building.

Jamaat-ul-Ahrar, a Pakistani Taliban-linked armed group, claimed responsibility for the attack, which also wounded at least 83 people, including media personnel covering the protest.

A spokesman for the group warned in a statement that the blast was "just the start".

Witnesses told Al Jazeera that the blast occurred near the Punjab assembly building when a suicide bomber on a motorcycle rammed into a police vehicle.

At least five police officers were killed in the attack, according to Mushtaq Sukhera, inspector general of police in Punjab province.

"It was a suicide attack. The bomber exploded himself when successful negotiations were under way between police officials and the protesters," Sukhera told reporters.

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Al Jazeera's Kamal Hyder, reporting from the blast site, said security forces had cordoned off the area on Mall Road, one of Lahore's main arteries.

"The explosion was heard for several kilometres. It was a very powerful explosive device," Hyder said, adding that a state of emergency had been declared in the eastern city.

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"A number of people has been moved to hospitals, and the death toll is also mounting," our correspondent said.

Nawaz Sharif, Pakistan's prime minister, condemned the attack and vowed to step up the fight against armed groups in the country.

"Terrorism isn't a novelty for us. Our story has been one of constant struggle against its grasp and a fight for the soul of Pakistan," he said in a statement.

"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."

Lahore was the site of an Easter Day bombing that killed more than 70 people in a public park last year.

Jamaat-ul-Ahrar claimed responsibility for that attack, as well as for a bombing at a hospital in Quetta that killed 74 people in August last year.