Pakistan has announced three days of national mourning after a suicide bomber linked to the Taliban killed at least 70 people and wounded more than 300 others outside a park in Pakistan's eastern city of Lahore.

The explosion took place in the parking area of Gulshan Iqbal Park, just a few feet away from some children's swings.

The area was crowded at the time because Christians were gathering to celebrate the Easter holiday.

The Taliban faction Jamaat-ul-Ahrar said it carried out the attack, adding that it was targeting Pakistan's Christian minority.

Prime Minister David Cameron has promised British help for Pakistan after the bombing, while Foreign Secretary Philip Hammond said British nationals were advised to avoid the area and monitor local travel advice and local media.

Many families were leaving the park when the blast happened.

Muhammad Usman, an administration official in Lahore, said more than 50 children were among the injured.

Eyewitnesses said they saw body parts strewn across the car park in the wake of the attack.

Television footage shows children and women crying as rescue officials, police and bystanders carry the injured to ambulances.

Javed Ali, a 35-year-old resident who lives opposite the park, said the force of the blast shattered his home's windows.

"Everything was shaking - there were cries and dust everywhere," he said.

"After 10 minutes I went outside. There was human flesh on the walls of our house. People were crying, I could hear ambulances."

Superintendent Mustansar Feroz said most of the dead are women and children.

Lahore is the capital of Prime Minister Nawaz Sharif's political heartland of Punjab, the biggest and wealthiest province in Pakistan.

The country is currently battling a Taliban insurgency, criminal gangs and sectarian violence.

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 attack both Pakistan and Afghanistan.

Sunday's blast came as the army was also deployed on the streets of Islamabad after thousands of protesters clashed with police in chaotic scenes, throwing stones and setting a container on fire.

The demonstrators were supporters of Islamist assassin Mumtaz Qadri, who was hanged on 29 February for killing a Punjab governor over his call for blasphemy reform.