At least 60 were killed, and hundreds more injured, in the blast in a local park, authorities reported.
A health adviser to the regional government said upwards of 280 people were injured and it was feared the number of dead would continue to rise throughout the night.
The blast occurred Sunday evening at the main gate to Gulshan-e-Iqbal park.
Many Pakistani Christians gathered in the park to celebrate Easter after church services.
The Taliban splinter group Jamaat-ul-Ahrar publicly claimed responsibility for the attack.
“The target was Christians,” said spokesman for the faction Ehsanullah Ehsan, who called multiple press agencies Sunday. “We want to send this message to Prime Minister Nawaz Sharif that we have entered Lahore.”
“He can do what he wants but he won’t be able to stop us. Our suicide bombers will continue these attacks.”
Security officials told the Dawn newspaper the blast was the work of a suicide bomber, whose head had been recovered. Police also said it appeared to be a suicide bomb.
Most of the dead and injured were women and children, a senior police officer toldReuters.
The Pakistani army was called in to control crowds around the park following the blast.
“The United States stands with the people and government of Pakistan at this difficult hour. We will continue to work with our partners in Pakistan and across the region … to root out the scourge of terrorism.” The U.S. is a strategic ally of Pakistan.
Facebook turned on its emergency check-in services to enable those in the area or who might know someone in the area to quickly let their friends and family know they are safe.
However, an error on the site caused alerts to be sent out to people all over the world, regardless of whether or not they had been to or knew people in Pakistan.
Punjab, of which Lahore is the capital, is Pakistan’s political center, as well as the largest and wealthiest province in the country. It is where Prime Minister Nawaz Sharif resides and functions.
United Kingdom Prime Minister David Cameron game his condolences over Twitter, saying the U.K. would “do what it can to help.”
PM: I'm shocked by the terrorist attack in Lahore. My thoughts are with the families & friends of the victims. We'll do what we can to help.
— UK Prime Minister (@Number10gov) March 27, 2016
Earlier on Sunday, the prime minister had shared a post on Facebook to celebrate Easter for “Christians all over the world and particularly the Christian community in Pakistan.”
“We are committed to ensure that all the communities in Pakistan enjoy equal rights irrespective of their beliefs,” he wrote. “We are determined to work for the uplift and welfare of all minorities so that they can contribute to the progress and development of the country like their Muslim brethren.”