UPDATED 11.29AM The death toll has risen to 65 in Pakistan, where a suicide bomber has struck at a public park in Lahore, filled with children.

More than 300 others, mostly women and children, have been injured.

A breakaway Pakistani faction of the Taliban, Jamaat-ul-Ahrar has claimed responsibility for the carnage and said it had deliberately targeted the Christian community.

A spokespeson for the faction said: "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."

Witnesses said they saw body parts strewn across the parking lot once the dust had settled after the blast.

"When the blast occurred, the flames were so high they reached above the trees and I saw bodies flying in the air," said Hasan Imran, 30, a resident who had gone to Gulshan-e-Iqbal Park for a walk.

Salman Rafique, a health adviser for the Punjab provincial government, said many of the wounded were undergoing emergency surgery in hospitals.

"We fear that the death toll may climb considerably," he said.

TV footage showed children and women standing in pools of blood outside the park, crying and screaming as rescue workers, officials, police and bystanders carrying injured people to ambulances and private cars.

Many of the injured were transported to hospitals on taxis and auto-rickshaws due to a shortage of ambulances, with hundreds of citizens arriving outside hospitals to donate blood.

"We were just here to have a nice evening and enjoy the weather," Nasreen Bibi said at the Services Hospital, crying as she waited for doctors to update her on the condition of her two-year-old injured daughter.

"May God shower his wrath upon these attackers. What kind of people target little children in a park?"

A university professor said the brand of terrorism in Pakistan is especially cruel.

Alexander Gillespie from Waikato University said terrorists there have targeted universities and school children in the past.

"They often do in fact attack children or religious festivities. It's a particularly cruel type of terrorism. All terrorism's cruel, but often the terrorism in Pakistan is particularly vicious".

Alexander Gillespie said a thousand people are killed a year in terrorist attacks in Pakistan.

The United States, a strategic ally of Pakistan, condemned the attack.

"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," White House National Security Council spokesman Ned Price said in a statement.

British Prime Minister David Cameron has promised help for Pakistan.

Cameron, who used his Easter message to urge Britons of all faiths to stand up for Christian values, said he was shocked by the attack.

"My thoughts are with the families and friends of the victims. We will do what we can to help," the PM posted on his Twitter feed.