One of Pakistan's best known Sufi musicians has been shot dead by unknown assailants riding a motorcycle in Karachi, triggering an outpouring of grief over what police described as an "act of terror".

Key points: The Sufi singer was driving to a television studio when he was shot

The Sufi singer was driving to a television studio when he was shot Amjad Sabri was the son of another legendary Qawwali singer

Amjad Sabri was the son of another legendary Qawwali singer Taliban known to have attacked Sufi mosques in the past

Amjad Sabri, 45, was travelling by car from his home in the city's eastern Korangi area to a television studio, when a motorcycle pulled up alongside the vehicle and the attackers opened fire, Farooq Sanjarani, a police officer said.

Sabri was hit by five bullets and was declared dead at Abbasi Shaheed Hospital while a companion, named as a relative, Saleem Sabri, was in critical condition, a hospital source added.

"It was a targeted killing and an act of terrorism," Muqaddas Haider, a senior police officer said, without naming possible suspects.

Grisly mobile phone footage of the scene of the crime shot by an onlooker showed the singer's head slumped on his right shoulder and a pool of blood on the ground by the driver's side where he sat.

Sabri was a "Qawwal", or singer of Qawwali, which is a traditional form of Islamic devotional music that is popular across South Asia with roots tracing back to the 13th century.

The music is closely associated with Sufism, a mystical sect of Islam that is viewed as heretical by hardline groups such as the Taliban.

The Taliban and other Islamist groups have carried out major attacks on Sufi mosques and shrines in recent years, including the 2010 bombing of the Data Darbar shrine in Lahore that killed more than 40 people.

The car that was being driven by Sufi singer Amjad Sabri. ( Reuters: Akhtar Soomro )

Sabri, the son of another legendary Qawwali singer, Ghulam Farid Sabri who died in 1994, was a fixture on national television and regularly performed on a morning show during the ongoing holy Muslim month of Ramadan.

In May 2014 he was asked by a court to respond to blasphemy charges following the broadcast of a controversial song-and-dance routine that was set to a Qawwali piece about the wedding of the Prophet Mohammed's daughter to his cousin.

Shock and condemnation

His killing was met with shock and condemnation. Neighbours congregated outside the singer's home to offer condolences to his relatives, while TV channels broadcast recordings of his music in tribute.

"Totally shocked to hear the news of @AmjadSabri. May Allah bless him with Jannah [heaven] for he praised Him & His Prophet beautifully all his life," tweeted Ayaz Sadiq, the speaker of Pakistan's Parliament.

"Shocked and saddened by news of the killing of Amjad Sabri, not just a crime but an attack on our culture and heritage," added Mustafa Qadri, a human rights researcher at Amnesty International.

While the motive behind the killing was not immediately clear, Arieb Azhar, another popular Sufi singer, said he believed Sabri may have been targeted because of his religious views.

"Our own dear Amjad Sabri, son of Ghulam Farid Sabri and nephew of Maqbool Sabri, the renowned Sabri brothers, was a true lover of God, life and all that's good," he said.

"His mission of love has tragically been cut short by those who spread hate in the world, and is a great loss for all the divided people of our country."

Karachi, a city of 20 million and Pakistan's economic hub, is frequently hit by religious, political and ethnic violence.

Paramilitary forces began a sweeping crackdown on militants in the city in 2013, which has led to a substantial drop in overall levels of violence.

AFP