Pakistan's President Pervez Musharraf has said UK investigators are to assist in the inquiry into the assassination of opposition leader Benazir Bhutto.
In a televised address, he said Prime Minister Gordon Brown had agreed to send a team of detectives from London to help establish what happened.
He said "terrorists" were behind the murder, and described Ms Bhutto's death as a "great tragedy" for the nation.
Elections set for next week in Pakistan have been postponed until 18 February.
Mr Musharraf said too much damage had been done to polling stations and voter papers during the unrest in the wake of Ms Bhutto's assassination last Thursday.
He said "miscreants and political elements" had taken advantage of the situation - "looting, burning and killing".
"Election commission offices, their centres, polling stations and their equipment were all damaged and destroyed. Hence the election commission was facing a big difficulty to hold these elections [on 8 January]," he said.
Mr Musharraf said he was setting up a commission to identify who was responsible for the widespread violence, in which at least 47 people have died.
At the same time, the president promised free, fair, transparent and peaceful elections.
He called for reconciliation rather than confrontation in the run-up to the polls, and said troops and paramilitary soldiers would ensure law and order during this time.
The 30-minute speech was Mr Musharraf's first major public address since Ms Bhutto's death.
Mr Musharraf referred to "the pain and anger" of Ms Bhutto's Pakistan People's Party (PPP), especially in her home province of Sindh.
He paid tribute to his political opponent, saying: "I also feel the same sadness and anger - I respect the sentiments of the nation."
He repeated official allegations that al-Qaeda was behind Ms Bhutto's killing, and urged the media to "expose" pro-Taleban militant leaders who, he said, were orchestrating suicide attacks in Pakistan.
He said new evidence was coming to light but that expert advice was needed, and he thanked the British prime minister for accepting his request for assistance.
"This is a very significant investigation. All the confusion that has been created in the nation must be resolved," Mr Musharraf said.
The UK's Metropolitan Police Service has confirmed that it is to send a small team from its counter-terrorism branch to provide support in the Bhutto murder inquiry.
In a statement, it said the officers would travel to Pakistan as soon as possible but that the Pakistani authorities would continue to lead the investigation.
Mr Musharraf says soldiers will keep law and order during elections
But the BBC's security correspondent Frank Gardner says that privately, British detectives are wondering how much they will be able to achieve in Pakistan as by now, nearly all forensic evidence has been lost from the crime scene and there is little usable footage of the attack.
Our correspondent says there is also the sensitive question of the exhumation of Ms Bhutto's body - something her family has so far opposed.
Unless her body is exhumed and examined by neutral experts it will be impossible to establish what actually killed her, he says.
Speaking shortly after the president's address, Ms Bhutto's widower, Asif Ali Zardari, said Britain's involvement in the investigation was too late, and should have been requested after the earlier attempt on her life in October.
Ms Bhutto's party has been demanding a wider, UN inquiry, and has criticised government efforts as woefully inadequate.
Electoral posters and banners still hang from buildings in Karachi
Mr Zardari confirmed the PPP would take part in the re-scheduled elections, saying: "It is the legacy of our leader Zulfiqar Ali Bhutto to participate in elections, no matter what the circumstances."
The BBC's Chris Morris in Islamabad says the PPP wanted elections as soon as possible, in order to take advantage of what could be a big sympathy vote.
The ruling PML-Q party had said the 8 January vote should be delayed for several weeks, on the grounds that the vote would "lose credibility" if held under current conditions.
The election is seen as a crucial move towards democratic rule under President Musharraf, an important ally in the US-led "war on terror" who stood down as army chief in November.