The attack took place in a region where Pakistani security forces are battling extremists linked to the Taliban and al-Qaida terror network.

Witnesses and officials say that the bomber targeted voters at a polling station in the Buner district during a by-election for Pakistani parliament's lower house, the National Assembly.

A large number of voters, political workers and security personnel were present inside the building which collapsed after the powerful blast.

Most of the deaths are said to have occurred instantly and hospital sources say the toll is expected to rise because a number of those wounded are in critical condition.

A senior member of the provincial ruling party, Aqil Shah, tells VOA by telephone rescue workers were searching through the rubble for survivors.

"A lot of people have died, the building collapsed and they have got a lot of people under the debris and they have to be removed," said Shah.

The remote Pakistani district borders the northwestern Swat valley, where security forces are conducting operations against al-Qaida and Taliban-linked militants.

A spokesman for the outlawed Tehrik-e-Taliban Pakistan group has reportedly claimed responsibility for Sunday's attack, saying it was carried out in retaliation for attacks on its fighters.

The latest violence comes as Pakistan has begun shifting troops away from the northwestern region toward its eastern border with India amid tensions over the Mumbai attacks.

India blames Pakistan-based Islamic militants for the bloodshed in its financial capital. Critics say that a reduction of troops in the northwestern Pakistani region near the Afghan border could lead to a rise in extremist violence there.