Pakistan’s return to democracy was dealt a significant blow yesterday when two explosions greeted a former prime minister Benazir Bhutto, on her return to Karachi from exile, killing more than 120 and wounding hundreds more, according to local police. Ms. Bhutto, who was traveling in an open truck, was unhurt.

The fate of the government of Pakistan, which is at the forefront of the war on terror and has nuclear weapons, is of immense concern to America. It is a powerful ally in the NATO campaign against Taliban forces and has mounted a number of sorties into the mountains in the north of the country, where the leader of Al Qaeda, Osama bin Laden, is believed to be hiding.

America has quietly encouraged Pakistan’s president and army chief, President Musharraf, to strike a deal to allow the return of Ms. Bhutto, 54, and her Pakistan People’s Party to take part in the parliamentary elections due in January in an attempt to calm the Islamic nation’s turbulent political climate.

Although no group immediately claimed to have planted the bombs, a number of Islamist groups, including terrorists who support Al Qaeda, issued death threats against Ms. Bhutto ahead of her homecoming. Pakistani intelligence sources told government officials there were three specific plots to kill Ms. Bhutto.

The assassination of Ms. Bhutto would be a coup for those who oppose her pro-Western views, her opposition to Islamist terror, and her power-sharing deal with General Musharraf, and who would like to bring to an end America’s extensive financial and military support for Pakistan. She was well aware she was putting her life in danger by returning to Pakistan to take part in the January elections.

“I am not scared. I am thinking of my mission,” she told reporters on her plane yesterday from Dubai, where she has been living since 1999. “This is a movement for democracy because we are under threat from extremists and militants.”

As she arrived on Pakistani soil for the first time in eight years, a tearful Ms. Bhutto kissed a Koran. “I am thankful to God. I am very happy that I’m back in my country. I was dreaming of this day,” she told Reuters.

The estimated 20,000 police deployed along the route appear to have been overwhelmed by the nearly 200,000 flag-waving, drum-beating supporters who crowded the streets and climbed trees to welcome Ms. Bhutto back to her hometown. Her motorcade took 10 hours to cover the few miles between the airport and downtown Karachi.

Ms. Bhutto, dressed in a green shalwar kameez and a white headscarf, the colors of Pakistan’s flag, defied police advice to remain behind her vehicle’s bulletproof glass and instead waved to the crowd from atop her reinforced steel truck. The midnight blasts shattered windows in the vehicle and set alight an escorting police car. Most of the dead were members of her party. At least 20 policemen and a Pakistani television cameraman were among those killed.

Ms. Bhutto, who has three children, was on her way to deliver a speech at the mausoleum of Mohammed Ali Jinnah, who founded Pakistan as an Islamic state at the time of the British withdrawal from India in 1947. After the explosions, she was safely removed to her Karachi home, Bilawal House. The bomb attack has highlighted Ms. Bhutto’s impending political and legal troubles.

In her agreement with General Musharraf, she agreed to return to fight elections with a view to becoming prime minister for the third time on condition that charges of corruption against her and her husband, Asif Ali Zardari, stemming from her two previous periods in office as premier — December 1988 to August 1990 and July 1993 to November 1996 — be dropped. She is accused of embezzling public money and hoarding it in Swiss bank accounts.

Pakistan’s Supreme Court is currently deciding whether it is lawful for General Musharraf to contest the presidential election he won on October 6. They will next consider whether the amnesty granted Ms. Bhutto by the military leader is legal. The constitution also would have to be amended if Ms. Bhutto is to become prime minister for a third time.

If the court rules against either of them, Pakistan would face political turmoil. General Musharraf urged Ms. Bhutto to delay her return until the court had come to its conclusions, but she ignored the suggestion.

The White House was quick to condemn the blasts. “The United States condemns the violent attack in Pakistan and mourns the loss of innocent life there. Extremists will not be allowed to stop Pakistanis from selecting their representatives through an open and democratic process,” a spokesman for the National Security Council, Gordon Johndroe, said.

The Oxford- and Harvard-educated Ms. Bhutto is no stranger to political violence. Her brother was shot dead in Karachi in 1996 and her father, Zulfikar Ali Bhutto, a former prime minister of Pakistan, was hanged in 1979 by the military dictator Muhammad Zia-ul-Haq.