The first and most deadly attack occurred in the eastern city of Lahore, where the bomber detonated his explosives as police tried to search him, killing 10 people. About an hour later, a second bomber struck in the southern port city of Karachi, killing at least two people.
CBS News' Farhan Bokhari in Islamabad reports that, according to some accounts, as many as 13 people died in Lahore and at least three people died in Karachi.
The attacks laid bare the challenges facing Pakistani officials trying to secure cities far from the northwest, where militants fighting Pakistan's U.S.-allied government and American forces in neighboring Afghanistan have long thrived. Many recent attacks have targeted minority Muslim and other religious groups.
The Taliban claimed responsibility for the attack in Lahore, where thousands of Shiite worshippers were marking the end of a 40-day mourning period for the Islamic sect's most beloved saint when the blast hit their procession early Tuesday evening.
A hardcore Islamic militant group known for its links to al Qaeda and the Taliban was at the center of the investigation, Bokhari reports.
A senior intelligence official in Lahore told Bokhari the attacks appeared to be the work of Lashkar-e-Jhangvi, a shadowy hardcore group of militant Sunni Muslims with a history of attacks on Shias.
"Recent intelligence reports had suggested that Lashkar-e-Jhangvi was preparing to carry out a bold attack," said the intelligence official, who spoke to Bokhari on condition of anonymity. "We know this group has built up its suicide attack capabilities. There are other indications which I can not discuss."
There was also speculation of the involvement of Fidayeen-e-Islam, an offshoot of Pakistan's Taliban movement. However, a second intelligence official, who spoke to Bokhari on condition of anonymity, said, "The Fidayeen-e-Islam could be a front for Lashkar-i-Jhangvi. We are examining all possible aspects."
Known as "chehlum", Tuesday marked 40 days since the anniversary of the death of Imam Hussain, the grandson of the prophet Muhammad, Bokhari reports. Hussain was massacred in the 7th century along with 72 of his companions in an epic battle in Kerbala in southern Iraq.
The bomber is believed to have been a young teenage boy who was wearing a suicide bomb jacket and also carrying a bag full of explosives, said senior police official Aslam Tareen.
A Western diplomat based in Islamabad who spoke Bokhari on condition that he would not be named because he was not authorized to speak to journalists said the profile of the suicide bomber in Lahore was particularly disturbing.
"This was a teenaged boy," the diplomat told Bokhari. "I have seen reports of him being as young as 15. This suggests these militants have the capability to indoctrinate young boys and use them to their advantage … these boys are made to believe they will end up in heaven and will remain in the company of concubines."
The bomber detonated his explosives when police tried to search him, Tareen said.
"It is a great sacrifice by the police officers who laid down their lives to protect innocent people," said Tareen.
Footage from the scene showed ambulances racing to the area and men carrying away victims. One young man whose arm was apparently hurt screamed as he was placed on a stretcher. A white car caught up in the explosion was largely destroyed, its hood twisted upward. A man lay wounded on the ground with two women and a child weeping beside him.
Dr. Zahid Pervaiz at the city's Mayo Hospital told reporters that 10 dead bodies had come in, while another 52 people were wounded.
Shakirullah Shakir, a spokesman for the Fidayeen-e-Islam wing of the Pakistani Taliban, told The Associated Press in a phone call that the militant group had dispatched the bomber and warned of more bombings.
Later Tuesday evening, a suicide bomber struck police protecting a group of people returning from a Shiite march in the Malir neighborhood of Karachi, Pakistan's largest city.
Three people were killed and three wounded in the attack, said Hamid Paryar, a doctor at Karachi's largest hospital. Two of those killed and one wounded were policemen, he said.
The third person killed is believed to be the suicide bomber, who was riding a motorcycle and carrying his explosives in a bag, said senior police official Shaukat Shah.
The blast occurred near a police van protecting the Shiite marchers, said Akbar Jaffry, who witnessed the bombing.
The conflict between Sunni and Shia Muslims has intensified in Pakistan during the past 30 years, Bokhari reports. It has been fuelled by support from Saudi Arabia to hardcore Sunni groups in the hope that they will suppress the Shias, who are seen as inclined toward Iran, the world's largest predominantly Shia country.
Attacks roughly tripled last year in Lahore and Karachi, according to a recent report by the Islamabad-based Pak Institute for Peace Studies.
The trend is a sign that militants are having greater success exporting the fight far from their northwest heartland along the Afghan border. The Pakistani army, under U.S. pressure, has carried out several offensives against militants in its northwest, but violence persists.