Islamabad, Pakistan — Asif Ali Zardari, the widower of the assassinated former prime minister, Benazir Bhutto, has been sworn into office as Pakistan’s president, completing a remarkable transformation from jailbird to head of state.

Mr. Zardari, 53, who was known as Mr. Ten Percent during his wife’s two spells of government due to unproven allegations of corruption, became Pakistan’s 12th and youngest president.

His assumption of office was hoped by most Pakistanis to herald a possible move towards steadying the government after a year of political upheaval in a country facing mounting security and economic problems.

Last month Mr. Zardari and his former coalition partner, Nawaz Sharif, forced the former military ruler, Pervez Musharraf, to resign as president after threatening him with impeachment.

Since his election by parliamentarians at the weekend the Pakistani press has been full of endless speculation over whether Mr. Zardari, whose name was a byword for corruption in the 1990s, has become “a reformed character.”

Editorials have mused over whether “leopards can change their spots” or people can “turn over a new leaf.”

However, few have been able to deny that as a pragmatist Mr. Zardari has played his cards deftly to achieve his goal in becoming Pakistan’s most powerful man.

Yesterday he swapped the well-laundered white robes that he has worn since his election victory on Saturday for a sombre pinstripe suit.

The gravity of his taking oath was underscored by a blast of trumpets that announced his arrival in front of Pakistan’s ruling politicians.

Mr. Zardari pledged to act “honestly” and not to let his “personal interest” sway his presidential judgment.

His three children, including the Oxford student, Bilawal, who is the chairman of the ruling Pakistan People’s Party, looked on as Mr. Zardari concluded his oath by beaming with his trademark Cheshire Cat smile.

Shouts of “Bhutto is alive,” rang out across a hall hung with portraits of Bhutto’s father, Zulfiqar Ali, and a pensive looking Allama Iqbal, Pakistan’s ideological-founder and dead national poet.

“It is a very significant day in the history of Pakistan and it is the victory of the people and the mission of Benazir Bhutto shaheed [martyr],” an aide to Mr. Zardari, Farzana Raja, said.

After taking oath Mr. Zardari was borne in a parasol-covered carriage, flanked by lance-bearing, mounted cavalrymen, to inspect a guard of honor.

The Afghan president, Hamid Karzai, who attended the ceremony, served as poignant reminder of why Mr. Zardari is in power: like Mr. Karzai, he is also in office in some good measure due to American and British support for leaders prepared to take on the Taliban and Al Qaeda.

Earlier this week President Bush anointed Mr. Zardari as the successor to Mr. Musharraf, as a key ally in the American-led “War on Terror.” He exhorted Pakistan to fight terrorism and said that he looked forward to working with Mr. Zardari in combating terrorism.

Mr. Zardari takes the reins of power as America starts an intensely belligerent phase of its campaign of targeting terrorist suspects on Pakistani soil.

The onslaught, which included America’s first ground assault in Pakistan’s tribal areas since 2001 last week, has fanned anti-American sentiment in Pakistan.