Pakistan's two major Opposition parties on Thursday demanded a UN-led inquiry into the killing of former premier Benazir Bhutto, claiming that the probe being conducted by the government is inadequate.

The demand was made by Bhutto's Pakistan People's Party and deposed Premier Nawaz Sharif's PML-N during an opposition convened session of the Senate, the upper house of Parliament.

PPP leader Raza Rabbani opened a debate in the Senate on the condolence reference on Bhutto's December 27 killing, saying the slain leader was aware of threats she was facing and had aired her fears during the last days of her life.

The senate session has been convened by the opposition to discuss sensitive issues like Bhutto's killing and a spate of militant attacks in a bid to corner the government.

The session was an emotional affair with several opposition leaders paying tribute to Bhutto. Rabbani broke down at one point and said talking about Bhutto's death was one of the most difficult things he had done.

Bhutto sacrificed her life for principles and democracy even though her opponents had termed her a 'security risk', Rabbani said, adding she was also the founder of Pakistan's missile programme and a great patriot.

The two-time premier was the symbol of the unity of the federation and did not discriminate among Sindhis, Punjabis, Balochis and Pashtuns, Rabbani said.

Referring to the government's investigation into Bhutto's murder, Rabbani said PPP did not accept this 'inadequate' probe and wanted a UN-led inquiry similar to the one into Lebanese prime minister Rafik Harriri's assassination.

Ishaq Dar of PML-N also backed PPP's demand for UN-led probe. The session was chaired by Jan Mohammad Jamali.