Foreign secretary David Miliband has said the killing of Pakistan opposition leader Benazir Bhutto was a "senseless attack".
Ms Bhutto, the former prime minister of Pakistan, was holding an election rally in Rawalpindi ahead of January elections when gunfire and an explosion occurred.
The two-time premier was shot in the chest and neck as she got into her car and the gunman then blew himself up.
In a statement, Mr Miliband said: "I am deeply shocked by news of the latest attack in Rawalpindi, which has claimed the life of Benazir Bhutto and killed at least 15 other people."
He continued: "Benazir Bhutto showed in her words and actions a deep commitment to her country. She knew the risks of her return to campaign but was convinced that her country needed her.
"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."
Mr Miliband added: "The large Pakistani community in the United Kingdom will be gravely concerned about these latest developments. Let me reassure them that the UK government will continue to work with all those who want to build a peaceful and democratic Pakistan."
Conservative party leader David Cameron described the attack as "an appalling act of terrorism".
He added: "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."
British prime minister Gordon Brown is to give his reaction to Ms Bhutto's killing later, while a White House spokesman said: "We condemn the acts of violence which took place today in Pakistan."
The United Nations security council is to hold an emergency meeting in the wake of the attack.