The motorcycle-borne bomber blew himself up when a security guard stopped him outside the office of National Database and Registration Authority (NADRA), that regulates government database and manages the sensitive registration database for the entire population of Pakistan, in Mardan town of the Khyber-Pakhtunkhwa, officials said.
"At least 26 people have been killed and more than 50 injured," said provincial information minister Mushtaq Ghani.
Up to 12 kilogrammes of explosive material may have been used in the blast that caused extensive damage to the office building, he said. The powerful explosion also shattered windows of several vehicles parked near the building.
He said the bomber belonged to the 22-25 age group. The attack took place at a time when some 400 people were present in the office which remains crowded most of the time.
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The injured, several of them critical, were shifted to Mardan Medical Complex, Peshawar hospital and other facilities in the city, where a state of emergency has been declared.
The area has been cordoned off as rescue and security officials rushed to the site. Interior Ministry has sought an immediate report on the blast.
Prime Minister Nawaz Sharif strongly condemned the attack and expressed deep grief over the loss of innocent lives.
Tehreek-i-Taliban Pakistan splinter group Jamaat-ul-Ahrar claimed responsibility for today's attack, saying it carried out the attack on the "heathen Pakistan state".
Today's bombing is one of the deadliest since a security crackdown following the Peshawar school massacre last year that saw 150 people, mostly schoolchildren, killed by the Taliban.