Al Jazeera's Imran Khan, reporting from the Pakistani capital, Islamabad, said: "This marketplace would have been packed. It's a place where families go to socialise. This underscores, yet again, how fragile and dangerous the situation in Pakistan is.
"Reports say there have been two separate blasts, but the pictures that we're seeing on local television here show mass fires breaking out ... it is likely the death toll will rise."
Kamal Hyder, Al Jazeera's correspondent also in Islamabad, said: "Immediately after the explosion, the entire area was plunged into darkness and rescue teams went in as fires burned throughout the shops.
The attack in Lahore was the third in the country on Monday, with one earlier in Peshawar, and a smaller one in Quetta.
At least seven people were killed in a suicide bomb attack outside a court in the northwestern city of Peshawar earlier during the day.
The bomber blew himself up at the gate to the court building after police stopped him, officials said.
Three of those killed were policemen. Dozens of other people were injured.
Peshawar, near the Afghan border, has been targeted repeatedly since Pakistan sent its troops to fight the Taliban in the tribal region of South Waziristan.
Also on Monday, eight people, including a child, were injured in a bomb attack in the southwestern Pakistani city of Quetta, police said.
The bomb, hidden in a motorcycle parked outside the gate of a government residential complex, was allegedly detonated by remote control, police said.
The Pakistani military said on Monday that its troops had killed four suspected fighters in a search operation in the northwestern Swat valley.
The army launched a successful offensive there in April and his since launched an offensive against fighters in the tribal regions along the country's border with Afghanistan.