At least 59 people were killed and 157 injured after an explosion in a mosque next to the Police Lines in Peshawar on Monday, with authorities fearing that the death toll could still rise.

The explosion took place when people gathered in the mosque for afternoon prayers.

Soon after the blast, a heavy contingent of police rushed to the spot and cordoned it off. Several ambulances shifted the injured to the Lady Reading Hospital as the city went into a state of emergency.

In an earlier update, Mohammad Asim, a spokesman for the Lady Reading Hospital in Peshawar, told Reuters they had received 90 injured, some of them in critical condition.

Reports indicated a part of the mosque was heavily damaged due to the explosion and several people were believed to be under it, resulting in higher casualties.

Footage showed police and residents scrambling to remove debris from the blast site and carrying wounded people on their shoulders.

The mosque is located in a highly fortified area of the city that houses police offices and residences.

Peshawar, which sits at the edge of Pakistan's tribal districts bordering Afghanistan, is frequently targeted by militant groups.

While talking to the media, Capital City Police Officer, Peshawar (CCPO) Ejaz Khan said that it was too early to say anything as many people are still trapped under the rubble. Between 300 to 400 police officials were present in the area at the time of the blast, he said.

"We cannot say with surety but there is a smell of explosives inside the mosque," said the CCPO.

An emergency was imposed in the locality. Islamabad Inspector General Dr Akbar Nasir Khan issued directions for a “security high-alert”. In a tweet, Islamabad Police said security at all entry and exit points of the city has been increased and monitoring was being done through the “Safe City” system.

Prime Minister Shehbaz Sharif strongly denounced the blast and said that targeting the mosque is proof that the attackers have nothing to do with Islam.

He said a comprehensive strategy will be adopted on the deteriorating law and order situation in Khyber Pakhtunkhwa, adding that the centre will cooperate in increasing the anti-terrorism capacity of the provinces, as per Radio Pakistan.

Following the incident, PM Shehbaz and Chief of Army Staff General Asim Munir visited Peshawar and were briefed on the blast.

Recounting the sheer scale of the tragedy, the premier said: "This is no less than an attack on Pakistan," adding that the "nation is overwhelmed by a deep sense of grief."

"I have no doubt terrorism is our foremost national security challenge," he said in a Twitter post.

"While the pain of the grieving families cannot be described in words, I express my heartfelt condolences and most sincere sympathies."

The premier said the perpetrators of today's "despicable incident can't undermine the resolve of our people."

Foreign Minister Bilawal Bhutto Zardari said, “terrorist incidents before the local and general elections were meaningful”.

In a tweet, Pakistan Peoples Party quoted Bilawal as saying that strict action will be taken against the terrorists, their patrons, and facilitators.

The FM called party workers and position-holders to donate their blood to save the lives of the injured.

Interior Minister Rana Sanaullah also took to Twitter and wrote that investigations of all aspects of the incident are underway.

"Federal institutions are fully supporting the Khyber Pakhtunkhwa government," he said.

In a tweet, PTI Chairman Imran Khan also strongly condemned the “terrorist suicide attack” in Peshawar and expressed sympathies with the families of the victims.

National Assembly Speaker Raja Pervaiz Ashraf and Deputy Speaker Zahid Akram Durrani also strongly condemned the explosion.

Pakistan Muslim League-Nawaz (PML-N) leader Maryam Nawaz also condemned the incident.

The US embassy in Islamabad extended its deepest condolences to the families of the victims of the horrific attack.

"The United States stands with Pakistan in condemning all forms of terrorism," the mission said in its message.

The attack was the city's worst since March last year when a suicide bombing at a mosque during Friday prayers killed at least 58 people and injured nearly 200. Islamic State militants claimed responsibility for that bombing.

UN Secretary-General condemns blast

UN Secretary-General Antonio Guterres on Monday condemned the "abhorrent" blast at a Pakistan mosque.

"The secretary-general strongly condemns the suicide bombing that took place at a mosque in Peshawar in Pakistan earlier today," said Stephane Dujarric, Guterres's spokesman. "It is particularly abhorrent that such an attack occurred at a place of worship."