Security forces are on high alert in Pakistan following the killing of opposition leader Benazir Bhutto at an election rally on Thursday.
Riots broke out in several cities after Bhutto died of injuries sustained in a shooting. She was buried on Friday.
Bhutto's killing has raised doubts about whether planned parliamentary elections can go ahead on January 8, as the government has stated.
Pakistani President Pervez Musharraf condemned the killing and called for calm.
But rioting which began on Thursday evening continued into Friday as angry Bhutto supporters took to the streets. At least 23 people were reported killed.
President Bush condemned the assassination while the United Nations secretary-general, Ban Ki-moon, called it an "assault on stability" in Pakistan.
The UN Security Council described Bhutto's death as a serious blow to stability in the region and demanded justice for what it described as "this reprehensible act". It called on all Pakistanis to exercise restraint.
Manmohan Singh, the prime minister of India - which has fought three wars against Pakistan, said "the subcontinent has lost an outstanding leader who worked for democracy and reconciliation in her country."
The Swiss foreign ministry expressed Switzerland's condemnation and said Bhutto's killing could endanger the fragile democratic process in Pakistan.
Switzerland, like several other countries, has also advised its citizens not to travel to Pakistan at the moment.
Return from exile
Bhutto's death comes only ten weeks after she returned to Pakistan from eight years of self-imposed exile.
She escaped an attempt on her life on her return in October when a suicide bomber attacked her entourage as she was being driven through Karachi. The attack killed around 140 supporters of Bhutto's Pakistan People's Party (PPP) and members of her security team.
Bhutto had been negotiating Pakistan's transition to civilian-led democracy with Musharraf who had granted her an amnesty in old corruption cases. But relations between the two of them had soured.
Charges Bhutto was facing in Switzerland were dropped following the news of her death.
She and her husband, Asif Ali Zardari, were accused of receiving multi-million dollar kickbacks in exchange for handing out a contract to a Swiss firm during Bhutto's second term as prime minister between 1993 and 1996.
Bhutto's lawyer in Geneva, Alec Reymond, was quoted by Associated Press as saying the investigation into Bhutto had now been closed. "The file is closed because the death of the suspect requires the termination of public proceedings," Reymond said.
A parallel investigation into Zardari continues.
The politician was born on June 21, 1953, into a wealthy landowning family. Her father, Zulfikar Ali Bhutto, founded the PPP and was president and later prime minister of Pakistan from 1971 to 1977.
After gaining degrees in politics at Harvard and Oxford universities, she returned to Pakistan in 1977, just before the military seized power from her father. She inherited the leadership of the PPP after his execution in 1979 at the hands of the military government.
Bhutto was first voted in as prime minister in 1988, but was sacked by the then-president on corruption charges in 1990. She took power again in 1993 after her successor, Nawaz Sharif, was forced to resign after a row with the president.
Bhutto was no more successful in her second spell as prime minister, and Sharif was back in power by 1996.
In 1999, both Bhutto and her husband were sentenced to five years in jail and fined $8.6 million for taking kickbacks from a Swiss company hired to fight customs fraud.
Bhutto was abroad at the time of her conviction and chose not to return to Pakistan.
In 2006 she joined an Alliance for the Restoration of Democracy with her arch-rival Sharif, but the two disagreed over strategy for dealing with Musharraf.
swissinfo with agencies
Swiss corruption case
1994 - SGS and Cotecna sign contracts with Pakistan.
1997 - Pakistan asks Switzerland for legal assistance.
1998 - Bhutto and her husband charged in Pakistan and Switzerland.
1999 - The couple are sentenced to five years in prison by a Lahore tribunal.
2001 - The Pakistan Supreme Court rejects the judgement.
2004 - The couple are charged with advanced money laundering.
2007 - Benazir Bhutto benefits from a corruption amnesty in Pakistan.