Pakistani security officials searched for clues Friday to identify the assassin of former Prime Minister Benazir Bhutto while her supporters burned train stations in rampages to protest her killing less than two weeks before a crucial election.

The death of President Pervez Musharraf's most powerful political opponent in a shooting and bombing attack plunged the nation into turmoil and threatened its already unsteady role as a vital bulwark against Islamic terror.

Angry Bhutto supporters ran amok through the streets after her assassination, lighting cars and stores on fire in violence that killed at least 10 people. The killing and its aftermath badly damaged plans to restore democracy in this nuclear-armed U.S. ally.

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Prime Minister Mohammedmian Soomro said Friday that the government had no immediate plan to postpone Jan. 8 parliamentary elections, despite the growing chaos and a top opposition leader's decision to boycott the poll.

"Right now the elections stand where they were," he told a news conference. "We will consult all the political parties to take any decision about it."

Bhutto was to be buried Friday afternoon in the family's ancestral village of Garhi Khuda Bakhsh, next to the grave of her father, also a popular opposition political leader who met a violent death, said Nazir Dhoki, a spokesman for Bhutto's party.

Bhutto's husband, Asif Ali Zardari, and her three children arrived from Dubai to attend the funeral, Dhoki said, sobbing with grief during a telephone interview.

Violent mobs burned 10 railway stations and several trains across Bhutto's Sindh province, forcing the suspension of all train service between the city of Karachi and the eastern Punjab province, said Mir Mohammed Khaskheli, a senior railroad official.

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The rioters uprooted one section of the track leading to the Indian border, he said.

About 4,000 Bhutto party supporters rallied in the northwestern city of Peshawar on Friday and several hundred of them ransacked the office of the main pro-Musharraf party, burning furniture and stationery. The office was empty and no one was hurt.

Protesters, carrying the green, red and black flags of Bhutto's Pakistan People's Party shouted "Musharraf dog" and "Bhutto was alive yesterday, Bhutto is alive today." Dozens of police in riot gear followed the protesters but did not intervene.

Protesters in Peshawar also burned the office of a small party allied with Musharraf.

Other areas were nearly deserted Friday morning as businesses closed and public transportation came to a halt at the start of three days of national mourning for the opposition leader.

Troops were put on alert in four cities in Sindh as a precaution against new violence, said army spokesman Maj. Gen. Waheed Arshad.

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A coalition of opposition parties called for a general strike, said Mohammed Usman Kakar, a leader in the All Parties Democratic Movement, which comprises small anti-Musharraf groups.

"The repercussions of her murder will continue to unfold for months, even years," read a mournful editorial in the Dawn newspaper. "What is clear is that Pakistan's political landscape will never be the same, having lost one of its finest daughters."

As many mourned Pakistanis mourned, others demanded answers as to who killed her.

Musharraf blamed the attack on the resurgent Islamic militants Pakistan is fighting along the border region with Afghanistan, pledging in a nationally televised speech that "we will not rest until we eliminate these terrorists and root them out."

But authorities said they had yet to identify the attacker.

"It is too early to say who may have been responsible," said Saud Aziz, the chief of police in Rawalpindi, the city near Islamabad where the attack took place. A joint task force of police and officials from other law enforcement agencies were investigating, he said.

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FBI spokesman Richard Kolko in Washington said the agency was trying to determine the validity of a purported claim of responsibility for the attack by al-Qaida.

U.S. President George W. Bush, who spoke briefly by phone with Musharraf, looked tense as he spoke to reporters, denouncing the "murderous extremists who are trying to undermine Pakistan's democracy."

In the wake of the killing, Nawaz Sharif, another former premier and leader of a rival opposition party, announced his party would boycott the elections.

The election was seen as a pivotal step toward restoring democracy here, eight years after Musharraf seized power in a coup. It also was intended to restore credibility to the government after Musharraf used a six-week state of emergency to arrest thousands of political opponents and crack down on the independent judiciary.

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However, with Sharif's party on the sidelines and Bhutto's party leaderless and in disarray, the election will have little if any credibility.

Sharif demanded Musharraf's resignation.

"Musharraf is the cause of all the problems," Sharif said.

Bhutto's death closed another grim chapter in Pakistan's bloodstained history, 28 years after her father, Zulfiqar Ali Bhutto, another ex-prime minister, was hanged by a military dictatorship just three kilometers from where she was killed.

The U.N. Security Council vigorously denounced the killing and urged "all Pakistanis to exercise restraint and maintain stability in the country."

The United States, meanwhile, struggled to reformulate its plan to stabilize the country based on a rapprochement between Bhutto and Musharraf.

At least 10 people were killed across the country in the violence following the assassination, including one police officer who died of his wounds early Friday. Bhutto's supporters in many towns burned banks, shops and state-run grocery stores. Some torched ruling party offices, Pakistani media reported.

Karachi police chief Azhar Farooqi said police arrested 40 people and increased street patrols to try to quell the violence.

Next to Musharraf, Bhutto, 54, was the country's best known political figure, serving two terms as prime minister between 1988 and 1996. She was respected in the West for her liberal outlook and determination to combat Islamic extremism.

Bhutto had just addressed more than 5,000 supporters in Rawalpindi on Thursday when the attacker struck as she was leaving the rally in a white sports utility vehicle.

A smiling Bhutto had stuck her head out of the sunroof to respond to youths chanting her name, said Sardar Qamar Hayyat, an official from Bhutto's party.

"Then I saw a thin, young man jumping toward her vehicle from the back and opening fire. Moments later, I saw her speeding vehicle going away. That was the time when I heard a blast and fell down," Hayyat said.

Bhutto was rushed into emergency surgery, but died from her gunshot wounds. Another 20 people were killed in the bombing, according to police and witnesses.