In the first attack in Lahore, a teenage suicide bomber blew himself up when he was challenged by policemen guarding a Shia procession.
The attack occurred at Lahori Gate near Karbala Gamay Shah, a Shia 'imambargah' or prayer hall.
Lahore police chief Aslam Tareen said the bomber, who was 14- or 15-year-old, detonated his explosive vest when he was stopped for checking.
Besides the vest, the bomber was carrying explosives in a bag, he said. The face of the bomber, minus the skull, had been found by police, he said.
"The attacker detonated his explosive vest at the first layer of security, where policemen were checking people (joining the procession), Superintendent of Police (Security) Faisal Rana told reporters.
Thirteen people, including a woman and four policemen, were among the dead. Over 70 people were injured.
Zahid Pervez, the medical superintendent of Mayo Hospital, described the condition of 10 of the injured as critical.
The second bomber struck a short while later, targeting a police van in the Malir area of Karachi. Two policemen were killed and five others, including three policemen, injured in the attack.
The attacker, who was on a motorcycle, rammed into the police van shortly after two buses carrying Shia Muslims returning from a procession had passed the area.
Police officials said the bomber struck after the van prevented him from hitting one of the buses.
Officials told the media they had found the head of the bomber and sent it for DNA and forensic tests.
In Lahore, police chief Aslam Tareen said authorities had received alerts of possible attacks on Shia processions. He said the multi-layered security arrangement put in place by police had prevented the bomber from reaching the procession.
Search and sweep operations were being conducted in the city to trace terror suspects, he said.
State-run Rescue 1122 spokesman Fahim Jehanzeb said that the blast in Lahore occurred minutes before the Shia procession reached the spot.
A suspect was arrested from the site of the blast, other officials said.
Shia Muslims across Pakistan observed the 'chehlum' of Hazrat Imam Hussain on Wednesday and a large procession that originated from the old quarters of Lahore was to culminate at Karbala Gamay Shah in Bhatti Gate.
The attack occurred despite extensive security arrangements put in place in major cities and towns to protect Shia processions. About 6,000 security personnel were deployed to guard processions in Lahore alone.
An emergency was declared in hospitals across Lahore and Karachi following the blasts.
President Asif Ali Zardari and Prime Minister Yousuf Raza Gilani condemned the attacks.