The explosion, which wounded more than 120, came at Wagah border gate near the eastern Pakistani city of Lahore after the "flag-lowering" ceremony, a display of military pageantry that attracts thousands of spectators every day and is popular with foreign tourists.

The attack is a rare strike in Punjab, Pakistan's richest and most populous province and powerbase of Prime Minister Nawaz Sharif, which has been spared the worst of the bloody wave of "Islamist" violence that has assailed the country in recent years.

"It appears to have been a suicide attack. At least 55 people have been killed and more than 120 wounded. Women and children were also killed," Mushtaq Sukhera, the Punjab provincial police chief, told AFP.

Lahore police chief Amin Wains confirmed it was a suicide attack."People were returning after watching the parade at Wagah border when the blast took place. Ball bearings were found at the scene," he said.

Huge crowds gather on both sides at Wagah each sunset to see the display of military pageantry that accompanies the formal closing of the border post. It appears the blast took place some distance from the border itself.

Tahir Javed, Punjab provincial commander of the Rangers paramilitary force that guards the post, said three of his men had been killed.

"The suicide bomber failed to cross the security barrier and blew himself up outside when people were coming out," he said.

There are several security checkpoints on the road leading to the border post, which is equipped with a ceremonial gate and banked seating, and spectators are frisked before entering, though such searches are not always particularly rigorous.

Conflicting claims
   
There were several conflicting claims of responsibility for the attack, reflecting the fragmentation the umbrella Tehreek-e-Taliban Pakistan (TTP) movement has undergone in recent weeks.

Abdullah Bahar, a spokesman for a TTP faction loyal to its dead chief Hakimullah Mehsud, said they carried it out to avenge Mehsud's killing in a US drone strike last year.

But the Jamat-ul-Ahrar faction, which broke away from the main TTP leadership in September, rubbished the claim and said they were behind the blast.

Group spokesman Ehsanullah Ehsan said in an email statement the attack was revenge for those killed in the ongoing military operation in North Waziristan tribal area, on the Afghan border. Ehsan said they would soon release video footage of the attack.

TV channels also ran claims from a third militant faction, called Jundullah.

Pakistan has been wracked by a homegrown Taliban insurgency that has killed thousands of people in recent years.

But attacks, once a near-daily occurrence, have tailed off since the army launched its anti-militant offensive in the northwest.

In June the army began the long-awaited operation against militant hideouts in North Waziristan after a bloody raid on Karachi Airport ended faltering peace talks between the government and the Taliban.

More than 1,100 militants and 100 soldiers have been killed since the start of the operation, and more than 100 militants have surrendered, according to the military.

The Pakistani authorities have also said they are becoming concerned about the possible spread of the influence of the ISIS group, which has declared a "caliphate" in parts of Iraq and Syria.

As well as being one of restive Pakistan's few tourist attractions, Wagah is also the main land crossing with India and much of their trade transits through it.

R.P.S Jaswal, deputy inspector general of India's Border Security Force which guards Wagah, said security has been stepped up along the Indian border although "our side is secure".

"A red alert has been issued keeping in view the blast across the border in ... Pakistan," Jaswal told AFP by phone from his base near the city of Amritsar.

The neighbours have had frosty relations since independence from Britain in 1947, fighting three full wars, two over the Himalayan territory of Kashmir.

Sunday's attack comes amid heightened tensions between the nuclear-armed neighbours after a resurgence of cross-border shelling along the disputed frontier in Kashmir.

It also comes with Pakistan on high security alert for the festival of Ashura, which is marked by minority Shiite Muslims with mourning processions that are potential targets for sectarian militants.

Last year saw deadly clashes erupt in the capital Islamabad's twin city Rawalpindi.