The mosque was nearly full when the attacker struck. By Tuesday morning, the toll stood at 87, the highest from a bombing in Pakistan in years.

The death toll from Pakistan’s bloodiest bombing in years, which tore through a mosque frequented by police officers in the city of Peshawar, rose to 87 on Tuesday morning, hospital officials said.

The suicide attack on Monday broke a period of relative calm in Peshawar, the capital of the restive Khyber-Pakhtunkhwa Province in northwestern Pakistan. The northwest has been the site of several attacks on police and military targets in recent months, especially in areas that straddle the border with Afghanistan, and the Pakistani Taliban have claimed responsibility for them.

On Monday, the group denied playing a role in this attack. Earlier, however, some junior commanders of the group claimed in Twitter posts that the bombing had been carried out to avenge the killing of a Taliban leader in a bomb blast in August in Afghanistan. No other group came forward to claim responsibility.

“It was a deafening explosion and I fell on the ground,” said Shabbir Afridi, 40, a government employee who was standing near the mosque at the time of the attack. “When I ran toward the mosque, I saw dust and bodies everywhere.”

The mosque was nearly full, with more than 300 worshipers filling the rows when the blast struck in Police Lines, a heavily guarded neighborhood that is home to several important government and military buildings. An office of the counterterrorism department is nearby, and officials said they were investigating how the suicide bomber managed to breach several security checkpoints to reach the mosque.

The roof caved in from the impact, trapping people under the debris.

“Most of the people are still trapped under the rubble. We fear that the number of casualties could increase,” said Akbar Khan, an official of the Edhi Foundation, a charity that runs a rescue service.

Many of the wounded were rushed to the nearby Lady Reading Hospital, a state-run medical facility, which was a scene of chaos. Doctors were struggling to move the many wounded to operating rooms while relatives thronged the hospital, frantically searching for information about their loved ones.

The hospital was put on alert, more medical personnel were called in, and people were asked to donate blood for the injured, according to hospital officials. By Tuesday morning, the number of wounded stood at 57.

The capital, Islamabad, was put on high alert, with snipers installed on important government buildings and checkpoints tightened on roads entering the city.

In the 1980s, Peshawar became one of the main staging grounds of local and international fighters in their struggle against the Soviet-backed Afghan government across the border. The city also saw a large influx of Afghan refugees.

The city has been scarred by terrorism over the past several decades, with a string of attacks targeting local government buildings and shopping areas. It was also the scene of the country’s most shocking terror event in December 2014, when Taliban militants attacked an army-run public school and killed 147 students and teachers. That attack sent major shock waves across the country and turned many in the public against the militants.

The last terrorist attack on Peshawar was also a suicide bombing, which targeted a mosque in March 2022 and killed more than 60 people. The Islamic State’s regional affiliate, Islamic State Khorasan, or ISIS-K, claimed responsibility.

“A large part of the KP province is in a state of war,” Khawaja Muhammad Asif, the country’s defense minister, told GEO TV, a local television news network. He said the security situation in the province had deteriorated significantly after the Taliban takeover of neighboring Afghanistan in 2021.

The Pakistani Taliban and the Afghan Taliban are separate entities, though they draw strength from some of the same ideological and religious beliefs. Several top Pakistan Taliban commanders have found safe haven in neighboring Afghanistan in recent years.

A peace deal between the Pakistani Taliban and Pakistan’s government broke down last year, and the resurgence of the Pakistani Taliban has strained relations between Pakistan and Afghanistan. The 2021 peace talks were initiated by the prime minister at the time, Imran Khan, whose government allowed many fighters to return from Afghanistan and resettle in the country if they pledged to lay down their arms.

Mr. Khan was ousted from government last year but has been making a spirited effort to return to power. His critics claimed that the peace talks only allowed the militants to regroup and strengthen.

Rights groups in Pakistan urged the government to take action against extremist groups.

“Had the state heeded earlier the warnings from civil society about the resurgence of extremist outfits in the province, it may have been possible to avoid this escalation,” the Human Rights Commission of Pakistan, a private rights group, said in a statement.

The police have been a major target of recent attacks across the Khyber-Pakhtunkhwa province. Police statistics show that 118 personnel were killed in militant attacks in 2022 in the province.

“We, the police, are the soft targets, and no one in Islamabad cares about us, ” said Muhammad, a police officer at a checkpoint in a suburb of Peshawar. He asked to be identified only by his first name out of fears for his security.

“Terrorists want to create fear by targeting those who perform the duty of defending Pakistan,” Prime Minister Shehbaz Sharif said in a statement.

The recent uptick in violence comes as Mr. Sharif is struggling to revive Pakistan’s ailing economy. The Pakistani currency fell to its lowest level against the dollar last week, and the government was forced to drastically increase fuel prices on Sunday in a bid to secure an International Monetary Fund bailout.

The I.M.F. has been pressuring the Pakistani government to cut government subsidies and make other changes to the structure of the economy.

“This is especially challenging for the country, as the surge in terrorist violence has come at a time when the economy is on the brink and politics is deeply polarized,” said Maleeha Lodhi, a former Pakistani ambassador to the United States. “Pakistan’s law enforcement agencies need to step up efforts to contain the surge in terrorist activity since the Taliban’s return to power in neighboring Afghanistan.”