The prime minister has paid tribute to Benazir Bhutto as "a woman of immense personal courage and bravery".

Gordon Brown said the killing of the former Pakistani prime minister was a "tragic hour".

Bhutto, Pakistan's opposition leader and twice previously prime minister, was killed in a suicide bombing on Thursday as she campaigned in the northern city of Rawalpindi, ahead of elections due in January.

She had spent many years in exile in Britain, only returning to her homeland earlier this year.

At least 16 other people died in the attack and several more were injured.

Brown said in a statement that: "This is a sad day for democracy. It's a tragic hour for Pakistan."

"Benazir Bhutto was a woman of immense personal courage and bravery," he added.

"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."

All the political parties in Britain agreed that the restoration of democracy in Pakistan must not be derailed.

Foreign secretary David Miliband added that he was "deeply shocked".

"This is a time for restraint but also unity," he said.

"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."

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."

And 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."