At least 15 people were killed and over 40 critically injured in two serial blasts in the southern Indian city of Hyderabad Thursday evening in the biggest terrorist attack in India since the 2008 Mumbai attacks, police said.

The serial blasts, which took place near a bus stop and a theater in a busy commercial area and near a police commissioner's office of the city, were no doubt a terrorist attack, police said.

Indian Prime Minister Manmohan Singh called the attack " dastardly" and vowed to punish the perpetrators of the blasts, while appealing for calm of the public.

Police in several other major cities of India, including capital Delhi and financial capital Mumbai, have been put on alert, according to official sources.

However, Indian intelligence officers also did not rule out other angles in the blast, which rocked the famous Dilsukh Nagar area known for its crowds.

Police rushed the injured to hospital and checked the sites of the blasts, according to local TV footages.

The bombs were planted on two motorbikes parked at the site of the explosion. Police have also found another IED bomb in downtown area.

The government denied charges of intelligence failure, although some hinted authorities got information that terrorists were ready to strike two days earlier.

Hyderabad, which has a large Muslim population, has been subject to terrorist attacks in the past.

Indian Muslims have been protesting the execution of militant Afzal Guru in the 2001 parliament attack over the past few weeks.

Relations between India and Pakistan have remained tense since last month after skirmishes broke out at the Line-of-Control in Kashmir, in which four Pakistani soldiers and two Indian soldiers were killed.