At least two people have been killed and over 20 others injured when a powerful bomb exploded outside a Shia mosque in Pakistan's largest city, police and hospital sources said.

According to reports on Sunday evening, the bomb was planted at the security barrier in the Gulshan-e-Iqbal area of Karachi.

Some reports suggested it was a remote control bomb which was fitted to a motorcycle.

Iqbal Mahmood, Additional Inspector General of Police for the Sindh province, confirmed the deaths of two people.

Fatehyab Jaffri, who was injured in the explosion, said he was sitting with others when a van stopped.

'People rushed for shelter'

"In front of the Suzuki van there was a motorcycle. As the Suzuki van stopped in front of a milk shop, the blast occurred. And people rushed for shelter," he said.

Karachi is in the grip of violence where ethnic, political and sectarian killings are common place.

Around 1,800 people have been killed in Karachi so far this year.

Officials said there were intelligence reports of "militant activities" in the city.

The attack came as Shias were celebrating the holy month of Muharram.

Pakistan has a long history of sectarian violence by extremist Sunni and Shia Muslims in Karachi, and other parts of the country.

Source: Agencies