Dozens of ambulances were seen racing to the park, situated near the centre of the city of around 8 million, with many women and children among the dead and wounded.
"The toll has risen to 56. The rescue operation is continuing," Lahore top administration official Muhammad Usman told AFP.
The army had been called in, he said, and soldiers were at the scene helping with rescue operations and security.
Senior police official Haider Ashraf said the blast appeared to be a suicide attack, adding that ball bearings were found at the crowded park.
A medical superintendent at Jinnah Hospital, who gave his name only as Dr. Ashraf, told AFP more than 40 dead bodies had arrived at the hospital.
"The number of injured stands at more than 200 people, most of them are in critical condition," he said. I fear the death toll will rise."
He described a nightmarish scene at the hospital, with staff treating casualties on floors and in corridors.
Javed Ali, a 35-year-old resident who lives opposite park, said the force of the blast had shattered his home's windows.
"Everything was shaking, there were cries and dust everywhere.
"After ten minutes I went outside. There was human flesh on the walls of our house. People were crying, I could hear ambulances."
He added: "It was overcrowded because of Easter, there were a lot of Christians there. It was so crowded I told my family not to go."
Pakistan has been battling a homegrown Islamist insurgency since 2004, with groups such as the Pakistani Taliban routinely carrying out attacks as part of their struggle to overthrow the government.
But Lahore, Pakistan's cultural capital that lies on the country's eastern border with India, has been relatively more peaceful in recent years.-------AFP