UK Prime Minister Gordon Brown has condemned the killers of Pakistan opposition leader Benazir Bhutto as "cowards afraid of democracy".
It was a "tragic hour" for Pakistan but terrorists must not be allowed to "win there, here or anywhere", he added.
Earlier, Foreign Secretary David Miliband called the apparent suicide bombing a "senseless attack".
The Tories said it was "an appalling act of terrorism" and the Lib Dems a "hammer blow" to democracy in Pakistan.
Ms Bhutto, a two-times prime minister, was killed as she campaigned in the northern city of Rawalpindi, ahead of elections due in January.
Mr Brown said: "This is a sad day for democracy. It's a tragic hour for Pakistan."
He added: "Benazir Bhutto was a woman of immense personal courage and bravery.
"Knowing, as she did, the threats to her life, the previous attempt at assassination, she risked everything in her attempt to win democracy in Pakistan, and she has been assassinated by cowards afraid of democracy.
"Benazir Bhutto may have been killed by terrorists, but the terrorists must not be allowed to kill democracy in Pakistan.
"And this atrocity strengthens our resolve that terrorists will not win there, here or anywhere in the world."
In a statement, Mr Miliband said he was "deeply shocked", adding: "This is a time for restraint but also unity.
"All those committed to a stable future for Pakistan will condemn without qualification all violence perpetrated against innocent people.
"In targeting Benazir Bhutto extremist groups have in their sights all those committed to democratic processes in Pakistan. They cannot and must not succeed."
The Pakistani flag flew at half-mast at the London embassy
Ms Bhutto, leader of the Pakistan People's Party, was holding an election rally when gunfire and an explosion occurred.
Conservative leader David Cameron said: "This is an appalling act of terrorism.
"Today Pakistan has lost one of its bravest daughters. Those responsible have not only murdered a courageous leader but have put at risk hopes for the country's return to democracy."
Liberal Democrat leader Nick Clegg said: "This is a dark day for everyone who believed in a stable and democratic future for Pakistan.
"Benazir Bhutto was a courageous politician known throughout the world, not just Pakistan, for her forthright and sometimes controversial views.
"Her tragic death is a hammer blow against the dream of pluralism and tolerance in modern day Pakistan.
"In the light of her brutal assassination, the need for the full restoration of democracy in Pakistan in now paramount."
And Justice Secretary Jack Straw said: "The sense of grief at her senseless killing will I know be shared not just by Pakistani communities across the world but by everyone, because this terrorism is an attack on us all, regardless of race, religion or party.
"We will ensure that it is defeated."
Conservative MP Rob Wilson, vice chairman of the all-party Parliamentary group for Pakistan, called for calm.
"It is important that when such a shocking event takes place, we don't rush quickly to premature judgements - we must wait for the facts to become clear."
He said many questions remained unanswered, such as who was responsible, whether Ms Bhutto had been warned about the attack and whether she had been well protected by the Pakistan government.
"The best tribute to Mrs Bhutto's memory will be the country coming together, and free and fair elections taking place," he added.