The death toll in the horrific suicide attack that shook Pakistan minutes after the popular flag-lowering ceremony at Wagah climbed to 61 today, even as security agencies arrested about 20 suspects from the Indo-Pak border area.

Ten women, eight children and three security personnel were among the 61 people who have died in the attack that took place yesterday when a suicide attacker detonated a powerful bomb at Wagah.

Over 100 people were injured in the incident and are being treated in different Lahore's hospitals where emergency has been declared.

The blast took place when a large number of people were returning after attending the flag lowering ceremony at the main Indo-Pak land border crossing, which is heavily guarded by the Punjab Rangers.

A young suicide-bomber blew himself up near the main exit gate of Parade Avenue at 6.03 pm (local time), causing colossal devastation.

"The death toll of the devastating suicide attack at Wagah border has risen to 61. The condition of some injured is still critical and the doctors are battling to save their lives," Punjab Emergency Services Rescue spokesman Jam Sajjad told PTI.

About 43 bodies have been handed over to the families while the remaining are yet to be recognised, Sajjad said.

"We have sought the help of the National Registration Database Authority (Nadra) for recognition of 18 bodies," he said.

The law enforcement agencies have launched operation in the residential areas in Wagah and taken about 20 suspects into custody, Lahore police spokesman Niyab Haider Naqvi said.

"A joint investigation team of police, rangers and intelligence agencies have started its probe into the incident. The army and rangers along with police will provide security to Ashura processions in Lahore and they have been put on high alert in the wake of Sunday's attack," he said.

Punjab Interior Minister Col (Retd) Shujah Khanzada said the target of the suicide bomber was the venue of the flag hoisting ceremony.

"The suicide-bomber wanted to reach the Parade Avenue to cause maximum devastation but he blew himself up near the entrance of the main gate after failing to reach the Parade Avenue," he said.

At least three militant factions have claimed the grizzly bombing. Al-Qaeda affiliated militant group Jundullah, a splinter group of the Tehrik-i-Taliban Pakistan (TTP), was first to claim responsibility of the attack.

Shortly afterwards Jamaat-ul-Ahrar said its bomber Hafiz Hanifullah carried out the attack. Later, a lesser-known Mahar Mehsud group also claimed responsibility for the bombing. According to a preliminary investigation report submitted to the Inspector General Police Punjab, Mushtaq Ahmed Sukhera, the suicide-bomber, believed to be in his early 20s, had been staying in the Wagah border area since many days.

"It is also suspected that the handlers of the suicide-bomber were also present near the blast site to ensure the execution of their plan," the report says.

At least 15 to 20 kilogrammes of explosives were used in the blast with some of the explosives in the attacker's suicide jacket and the remaining being carried by him.

The blown-up parts of the bomber have been collected and sent for DNA and forensic tests.

Meanwhile, Jammat-ud-Dawah's subsidiary Falah-e-Insaniat shifted nine dead bodies to its headquarters at Chauburji and one of its leaders led the funeral prayers for the deceased before handing over the bodies to the families.

"We have allowed the Falah-e-Insaniat to take the bodies of nine victims to Jamia Masjid Al-Qadsia Chauburji on the request of their families as they wanted to offer their funeral prayers there," Lahore's Mayo Hospital official Maqbool Ahmed said.

Punjab Law Minister Mujtaba Shujaur Rehman has termed the suicide blast a "security lapse".

"Carrying out a suicide blast at the Rangers headquarters is definitely a security lapse and very much alarming," he said, adding that the investigation agencies would have to look into how the suicide bomber managed to cross the security tiers and reached near the main entrance of the Parade Avenue.

A warning had been issued to the security agencies five days ago that "suicide bombers might target their buildings," an official of the home department told PTI.

"It seems all the focus of security has been shifted to protect Muharram processions and Wagah border became a soft target of the terrorists," he said.

An intelligence report had mentioned anti-Shia banned outfits -- Tahrik-i-Taliban Pakistan (TTP) and Lashkar-e- Jhangvi - might target Shia processions, important personalities and buildings of security agencies.

There would be no flag-lowering ceremony at Wagah for the general public for three days, a Punjab Rangers spokesman said.

"The decision has been taken to mourn the killings of the Sundays attack," he said, adding that India's BSF also told the rangers that it would also not invite the public for the ceremony for three days.