WATCH: The twin bombings were caught by security cameras at a Sufi shrine in Lahore.
Police and other security personnel are patrolling many key points of Lahore today over fears of new extremist attacks.
Security is particularly high at mosques, where crowds will gather for weekly prayers in the city.
The clampdown comes a day after two suicide bombers killed more than 40 people and wounded another 120 in an attack on a religious site extremists consider un-Islamic.
The attack targeted worshippers as they gathered in large numbers at the white marble mausoleum of a Sufi saint, Data Gunj Bakhsh.
One suicide bomber detonated his explosives in a big underground room where visitors sleep and wash before praying. The second blew himself up minutes later in an outdoor yard as people fled the first attack.
"There was chaos everywhere when this blast occurred," one survivor, Jamshed, told Reuters. "No one was there to help. Everyone was running to save their own lives. Some people ran out of gates, some ran toward the female portion of the shrine. Everyone was trying to save their own lives. No one was helping anyone. I saw many dead bodies in a pool of blood in the courtyard."
The shrine's caretaker, Mian Mohammad Munir, said there were at least 2,000 to 2,500 people in the shrine when the twin attacks took place.
A member of the shrine's volunteer security force intercepted one of the attackers but was unable to keep him from detonating his explosives-filled vest.
"These people don't have this written on their foreheads [that they are terrorists]," said Hajji Mohammad Ishaque, an eyewitness from the shrine administration. "One of our volunteers tried hard [to catch one attacker]. The cameras have the record. He attempted to catch him. One [of our colleagues] lost his life. Look, whatever you do, the one who has strapped bombs to his body is determined to die."
Today, some returned to the shrine to pray for those killed or wounded the day before.
Muslim Sects Targeted
No group has claimed responsibility for the attack but blame has focused on extremist groups with links to the Taliban and Al-Qaeda.
Extremist groups have killed some 265 people in Lahore in nine attacks since March of last year. Many of the attacks have targeted Muslim sects that radical groups brand heretical.
Sufisim is a mystical movement that includes both Sunni and Shi'a members and spreads its message of Islam through music, poetry, and dancing. Extremist groups with beliefs close to those of the Sunni fundamentalist Taliban consider music and dancing irreligious.
The July 1 attack on the Sufi shrine comes just weeks after suicide bombers and gunmen hit two prayer halls of another sect that extremist consider non-Islamic: the Ahmadis. The May 28 attacks killed 82 worshippers.
Pakistani authorities condemned the July 1 bombings as trying to intimidate mainstream Muslims and destroy religious tolerance in the country.
A spokeswoman for President Asif Ali Zardari, Farahnaz Ispahani, said "this sickening poison of extremism will be driven out of our nation and we will not be cowed."
Both the United States and the UN also condemned the attacks.
In a statement, U.S. Secretary of State Hillary Clinton said, “The extremists have shown that they respect neither human dignity nor the fundamental religious values of Pakistani society.”
A spokesperson for UN Secretary General Ban Ki-moon called the attacks “particularly vicious” for targeting a crowded place of worship.
Lahore, the capital of Punjab Province, is the cultural hub of Pakistan and home to many top figures in the country's military and business elite. It has become a frequent target for radical groups waging war on Pakistan's establishment in a loose alliance with Taliban militants in the northwest tribal areas.
The attacks, which include bombings of markets and even an assault on Sri Lanka's visiting cricket team in March 2009, have increased as the Pakistani Army undertakes a U.S.-backed campaign to destroy militant strongholds in the tribal areas.
The extremist groups operating in Punjab – Pakistan's wealthiest and most populous province – are widely referred to as the "Punjabi Taliban" due to their links with Taliban groups in the tribal areas and Afghanistan. Many were started with government support in the 1980s and '90s to fight the Soviets in Afghanistan but were later banned for causing unrest in Pakistan.
written by Charles Recknagel, with agency reports