The assassination of Benazir Bhutto threatens to plunge a nuclear armed Pakistan into civil war, opposition groups warned today.
The former prime minister was killed by a suicide bomber as she left a public rally in the city of Rawalpindi today.
Commentators said the killing would make the postponement of the upcoming elections almost certain and could spark further unrest.
Riaz Malik, of the opposition Pakistan Movement for Justice party (Pakistan Tehreek-e-Insaf), warned: "The impact will be that Pakistan is in more turmoil - it will be the start of civil war in Pakistan.
"There is a very real danger of civil war in Pakistan."
He said that while suspicion was likely to fall on insurgent groups based in the northern tribal areas near the Afghan border, the killing was bound to increase dissatisfaction with the regime of President Pervez Musharraf.
"There will be a lot of fingers pointed at the government," he said.
Pakistan has a short, but troubled history. East and West Pakistan were created in 1947 in frontier areas of British India, where the military had always been a key part of local administrations.
The generals would continue to play a significant role in the largely Muslim states.
The new leaders of East and West Pakistan found the early years hard and the difficulties in creating a constitution that could unite the various provinces were complicated by war with India in 1947.
The new parliamentary system was soon in difficulty. In 1958, General Ayub Khan launched the first of many military coups.
Martial law was declared, political parties abolished and a pattern of military control was established that has characterised almost half of Pakistan's existence since independence.
In 1960 General Khan became president. But a failure to win a second war with India in 1965, mounting corruption and increasingly uncontainable Bengali frustrations in East Pakistan gradually undermined Khan's authority, finally forcing his resignation in 1969.
The first election on a nationally democratic basis was conducted in 1970.
The elections saw the East-Pakistan Awami League gain an overall majority, which the West Pakistan administration refused to accept.
This set the stage for a new constitutional crisis that in turn led to civil war in March 1971 and, following Indian intervention, the emergence of East Pakistan as the independent state of Bangladesh.
Zulfiqar Ali Bhutto, Benazir Bhutto's father, was sworn in as prime minister in 1973.
In 1977 riots erupted over allegations of vote-rigging by Ali Bhutto's Pakistan People's Party. General Zia ul-Haq staged a military coup, becoming president and imposing martial law.
In 1979 Ali Bhutto was hanged on charges of the murder of the father of a dissident PPP politician.
In 1980 the United States pledged military assistance to Pakistan following Soviet intervention in Afghanistan.
General Zia demanded the expulsion of Russian forces from Afghanistan and gave asylum to Afghans. The huge amounts of aid that poured in propped up Zia's government.
Benazir Bhutto returned from exile in 1986 to lead PPP in campaign for fresh elections.
In 1988 General Zia died along with senior military officials in a mysterious air crash when his military aircraft exploded in mid-air a few minutes after take-off. The cause of the crash was not known and the inquiry report was never made public.
In the same year Benazir Bhutto's PPP won the general election but two years later she was dismissed as prime minister by the president, Ghulam Ishaq Khan, who exercised his power through a controversial constitutional amendment.
The president resigned in 1993 under pressure from the military and a general election returned Benazir Bhutto to power.
She was dismissed amid corruption allegations in 1996 and in 1999 the army, led by General Pervez Musharraf, once again ousted the civilian government.
The coup was widely condemned and Pakistan was suspended from the Commonwealth.
Aside from the political wranglings there was the continuing conflict with India.
Pakistan conducted nuclear tests in 1998 after India exploded several devices and then in 1999 Pakistan-backed forces clashed with the Indian military in the heights around Kargil in Indian-held Kashmir.
More than 1,000 people were killed on both sides.
Following the September 11 2001 attacks on the US, General Musharraf backed George Bush in the fight against terrorism and supported attacks on Afghanistan.
In recent years relations with India have warmed somewhat, but Pakistan remains a nation facing many challenges.
In March, General Musharraf suspended the Chief Justice Iftakar Mohammed Chaudhry, a move that triggered a wave of anger across the country and led to the first joint protests held by the parties of exiled former prime ministers Benazir Bhutto and Nawaz Sharif.
Then in October General Musharraf signed a corruption amnesty, opening the way for Ms Bhutto's return and a possible power-sharing agreement.
However, within hours of arriving back in the country bombers attacked a Bhutto rally in Karachi, killing more than 100 people.
Then in November, General Musharraf declared a state of emergency, rounding up opposition leaders at gunpoint.
Earlier this month, Musharraf lifted the state of emergency and announced plans to go ahead with parliamentary elections scheduled for January 8.
There were major doubts today about whether those elections would now go ahead.