Islamabad,  (IANS) Twenty-five people were killed and 80 injured Friday in two powerful bomb explosions in Pakistan's Karachi city, media reports said.

The first blast took place when a bus hit an explosives-laden motorcycle on a bridge on the Faisal highway. Thirteen people were killed and 60 injured in the attack.

Police officer Waseem Ahmed said the bomb was planted on the motorcycle and the blast was so powerful that it was heard from several kilometers away.

Sources said the bus was on its way to a Muharram procession when the blast took place.

Officials said 20 kg of explosives were used in the blast. The area was cordoned off after the blast.

The second blast took place at the Jinnah Post-Graduate Medical Centre where the injured of the first blast were brought. Twelve people were killed and many others injured in the attack, the Online news agency reported.

The blast took place at the gate of the hospital's emergency ward.

More than 200 people were outside the hospital when the blast took place. An eyewitness said the bomb was planted on a motorcycle parked inside the hospital compound.

Several ambulances were destroyed in the blast. The hospital employees were safe.

After hearing the news of the two blasts, the participants of a Muharram procession have staged a protest demonstration on M.A. Jinnah road.

Meanwhile, Shia Ulema Council has announced a three-day mourning in Karachi.

President Asif Ali Zardari and Prime Minister Yousuf Raza Gilani have strongly condemned the blasts in Karachi and ordered investigation into the incident.

Both leaders have urged people to remain calm.