ISLAMABAD, Pakistan — The widower of assassinated Pakistani leader Benazir Bhutto took office as the country’s new president today, facing immediate pressure to crack down on Islamic militants and address daunting economic problems.

Pakistan’s top judge swore in Asif Ali Zardari at a brief ceremony in the presidential palace recently vacated by Pervez Musharraf, who resigned under pressure last month.

With his three children among the well-wishers and dignitaries packing a cavernous hall, Mr. Zardari, wearing a pinstriped business suit, beamed as the ceremony ended and shouts of “Bhutto is alive!” rang out.

But in the front row sat an imposing reminder of his task ahead: Afghan President Hamid Karzai, whose government accuses Pakistan of failing to take action against — even colluding with — Taliban militants based around the countries’ common border.

“We are in the eye of the storm,” Mr. Zardari told a joint news conference with Mr. Karzai later in the day. “I consider that an opportunity. I intend to take that and make it our strength. We intend to take the world with us in developing the future of Pakistan and changing the future of our neighbors also.”

Mr. Karzai said Pakistan’s new democratic dawn augured well for both countries and that he found common ground with Mr. Zardari.

“For each step that you take in the war against terrorism for bringing peace to two countries, for bringing stability to two countries, Afghanistan will take many, many steps with you,” he said.

The inauguration of Mr. Zardari, 53, completes Pakistan’s return to civilian rule nearly nine years after then-army chief Mr. Musharraf seized power in a bloodless military coup.

America came to depend heavily on Mr. Musharraf for cooperation to capture or kill al-Qaida leaders who plotted the 9/11 attacks on America and fled Afghanistan after the American-led invasion in 2001 toppled their Taliban allies.

However, the Taliban revived on Mr. Musharraf’s watch, and al-Qaida chiefs Osama bin Laden and Ayman al-Zawahri remain on the run, probably somewhere in the uncontrolled frontier region.

Mr. Zardari has made tough statements against Islamic extremism, and the army says it has killed hundreds of rebels in ongoing operations in several parts of Pakistan’s volatile northwest.

The Pakistani Taliban have responded with a string of suicide bombings, including one in the city of Peshawar that killed 35 people Saturday, the same day as the presidential election by lawmakers.

Meanwhile, a rare assault by American ground troops and a series of missile strikes into Pakistan’s tribal region indicate that Washington is getting more aggressive about militant havens just beyond the Afghan border, despite intensifying Pakistani protests.

Officials said today the death toll from the latest missile strike had risen to 20 after residents and militants pulled more bodies from the rubble of a seminary and houses in a village in the North Waziristan region. Two Pakistani intelligence officials said the total included four suspected foreign militants.

At Washington, a State Department spokesman, Sean McCormack, wouldn’t say whether there had been a dramatic escalation of coalition operations on the border.

He said Washington looked forward to working with Mr. Zardari and his government on counterterrorism.

“They have a healthy appreciation for what’s at stake here. And we have good cooperation with this government,” Mr. McCormack said.

In the latest fighting, seven militants were killed today at northwestern Bajur region. Additionally, six civilians, including three children, died when mortar shells hit two houses overnight in the same region, officials said. It was not clear who fired the mortar rounds.

Yet the elected government also has sought peace talks with militants, and many Pakistanis blame the rising violence in their own country on Mr. Musharraf’s close alliance with Washington.

Mr. Musharraf quit reluctantly on Aug. 18 to avoid the threat of impeachment at the hands of a coalition of parties that routed his supporters in February parliamentary elections.

Mr. Musharraf made himself deeply unpopular by imposing emergency rule and purging the Supreme Court in November in order to halt legal challenges to his continued rule.

The ruling coalition, led by Mr. Zardari’s Pakistan People’s party, has collapsed over the failure to restore the judges.

But Mr. Zardari quickly found new allies, raising hopes that he can give Pakistan a degree of political stability, despite his lack of proven leadership skills and a reputation sullied by unproven corruption allegations.

Mr. Zardari won a two-thirds majority when lawmakers chose among the three presidential candidates on Saturday.

Ordinary Pakistanis are calling on the government to give them some relief from runaway inflation and massive power shortages. Economists are calling for urgent action to address slowing growth and investment plus fast-depleting foreign currency reserves.