The blast occurred in the parking area of Gulshan-e-Iqbal Park, metres away from children’s swings. More than 300 people were injured in the explosion.
There was no immediate claim of responsibility for the blast.
Pakistan, a nuclear-armed nation of 190m people, is plagued by a Taliban insurgency, criminal gangs, and sectarian violence. Punjab is its biggest and wealthiest province.
Eyewitnesses said they saw body parts strewn across the parking lot once the dust had settled after the blast.
The park had been particularly busy yesterday evening due to the Easter holiday weekend. There is speculation that Christian families out for the Easter weekend may have been the target.
Salman Rafique, a health adviser for the Punjab provincial government, put the death toll at a minimum of 52 people.
“Most of the dead and injured are women and children,” said Mustansar Feroz, police superintendent for the area in which the park is located.
Media footage showed children and women crying and screaming and rescue officials, police, and bystanders carrying injured people to ambulances and private cars.
One eyewitness said there was chaos, with a stampede breaking out and children separated from their parents in the rush to escape.
The explosion appears to have been at the main gate to the Gulshan-e-Iqbal park in an area where cars are usually left.
Senior police official Haider Ashraf said the blast appeared to be a suicide attack, adding that ball bearings were found at the crowded park.
In 2014, Pakistan launched an offensive against Taliban and affiliated jihadist fighters in North Waziristan, seeking to deprive them of safe havens from which to launch attacks in both Pakistan and Afghanistan.
Punjab has traditionally been more peaceful than other parts of Pakistan. Sharif’s opponents have accused him of tolerating militancy in return for peace in his province, a charge he strongly denies.
Last year saw the lowest number of civilian and security forces casualties since 2007, the year the umbrella Pakistani Taliban group was formed.
However, militants are still able to carry out major attacks from time to time.
At least 16 people were killed and more than two dozen wounded when a bomb blew up inside a bus in Peshawar, the main city of Pakistan’s insurgency-wracked north-west on March 16.
Last year, a bomb killed a popular Pakistani provincial minister and at least eight others when it destroyed the minister’s home in Punjab.