The attack left over 55 injured, several of whom were being treated at the Hayatabad Medical Complex. Another body was brought to the hospital later in the evening, raising the death toll to 20.
The attackers first hurled hand grenades before entering the Imamia Masjid Imambargah where they opened fire and carried out the first suicide-bombing.
Inspector General Police (IGP) Khyber Pakhtunkhwa Nasir Durrani told reporters the attackers entered the mosque by scaling a wall of an adjacent under-construction building. The IGP said the attackers arrived in a car which they burnt.
IGP Durrani added that the first suicide bomber carried out the attack in the gallery of the mosque while the body of the second suicide bomber was recovered from inside the mosque.
According to the IGP, people offering their prayers stopped a third suicide bomber from detonating his vest, though he was also killed in exchange of gunfire.
In-charge Bomb Disposal Squad (BDS) unit Shafqat Malik said the attackers looked like Uzbeks.
Earlier, SSP Operations Peshawar, Dr. Mian Saeed said at least three suicide bombers attacked the Imamia Masjid Imambargah. He confirmed that one suicide bomber detonated his explosives while another was injured and succumbed to his injuries, adding that his suicide-jacket had been defused.
An eyewitness of the attack told Geo News that the first explosion took place just minutes before Friday prayers were over. Following the first blast, five armed men in FC uniforms entered the imambargah and started to fire indiscriminately, the eye witness claimed.
Prime Minister Nawaz Sharif expressed grief over the loss of lives in the terrorist attack. The prime minister directed authorities to ensure provision of the best medical treatment to the injured. He said the government was committed to eradicating the menace of terrorism and extremism from the country.
Pakistan Tehreek-e-Insaf (PTI) Chairman Imran Khan condemned the attack and said he wanted to visit the site but was stopped by the police due to security concerns.
The Balochistan Shia conference has called for three days of mourning and called for the arrest of those involved in the attack within 48 hours.
The attack comes two weeks after a suicide bombing at an imambargah in Shikarpur, Sindh killed 61 people.
An attack carried out in a similar manner claimed the lives of over 150 people the majority of them school children when terrorists attacked the Army Public School in Peshawar on December 16, 2014.