More than 5,000 people have been arrested following Sunday's suicide bombing in Pakistan, a provincial minister has said.

73 people were killed and hundreds more injured in the blast in Lahore. Many of the victims were children.

Of the thousands arrested, around 200 people remain in custody.

"More than 5,000 people were searched and interrogated and most of them were allowed to go, but some 216 have been apprehended for further investigations," Rana Sanaullah told reporters.

The bombs went off near a children's play area in the park on Sunday afternoon as families were out enjoying the warm day.

The attack was claimed by the Jamaat-ul-Ahrar faction of the Tehreek-i-Taliban Pakistan (TTP).

The militants have since sent a taunting tweet to Pakistan's Prime Minister Nawaz Sharif, saying war has "reached his doorstep".

Ehansullah Ehsan, spokesman for the Jamaat-ul-Ahrar, tweeted: "After the Lahore attack, Nawaz Sharif repeated old words to give himself false assurances.

"Nawaz Sharif should know that war has reached his doorstep, and God willing the mujahideen will be the winners in this war."

The tweet was received as parks in the eastern city of Lahore were reopened amid tight security, although the Gulshan-i-Iqbal Park where the attack targeting Christians took place is still closed.

Kashir Nawab, a 32-year-old Christian from the Youhanabad neighbourhood of Lahore, said a "pall of gloom" hung over the area as mourners visited the homes of those lost in the blast.

"Everybody is frightened and the Christians particularly feel unprotected," he said.

The death toll from Sunday's attack has reportedly reached 73, with hundreds of others injured.

A 16-year-old teenager was one of the victims who died of his injuries. 

"The boy had lost his father and sister in the blast while his mother is being treated for critical injuries," a hospital doctor revealed.

In a televised address on Monday, Pakistan's PM pledged to avenge the attack, which was carried out in an area from which his ruling Pakistan Muslim League gains strong support.

"Terrorists cannot dent our resolve. Our struggle will continue until the complete elimination of the menace of terrorism," he said.

The attack on the Christians - who account for 1.6% of Pakistan's 200 million people and often complain of being discriminated against by the Muslim-majority - is the worst the country has seen this year.