At least 18 people were killed and 52 wounded when bombs ripped through crowded areas of the Indian city of Hyderabad on Thursday in what the prime minister called a "dastardly act".

The bombs targeted a mainly Hindu district in a suburb of the city, a hub of India's information-technology industry which has a large Muslim population, and came with the nation on alert after the recent hanging of a Kashmiri separatist.

"We have 18 people dead," a police officer who declined to be named told AFP.

Another senior police officer at the scene of one of the explosions, Amit Garg, put the number of wounded at 52.

Police said many of the injured were in critical condition in hospital.

"This is a dastardly act and the guilty will not go unpunished," Prime Minister Manmohan Singh said of the attacks, the deadliest to hit India since 13 people died in a 2011 bombing outside the High Court in the capital New Delhi.

But Singh also appealed for "calm" in the aftermath of the Hyderabad blasts.

City police said there had been three explosions, but Home Minister Sushil Kumar Shinde said he could only confirm two.

"The two bombs were placed on two different bicycles and the distance between them was about 100 to 150 metres (yards)," Shinde told reporters in New Delhi.

He said Indian authorities had received "intelligence inputs in the (recent) days about the possibility of attacks and this information was shared with other states".

Police said the blasts went off in quick succession.

Huge crowds gathered near the site of the explosions in the Hyderabad suburb of Dilsukh Nagar as police struggled to collect evidence.

"Ambulances have been rushed to the spot. Bodies have arrived and over 50 injured people have been brought to the spot," Kailash Nath, an officer at the Osmania General Hospital, told AFP.

At the hospital, bloodied victims lay on stretchers as sobbing relatives pleaded for information about their loved ones.

Nath said that nine bodies had arrived at the hospital and 35 people were undergoing surgery.

The blasts came on the same day as India's parliament opened for its key budget session, amid tensions following the hanging earlier this month of the Kashmiri separatist, Mohammed Afzal Guru.

The execution of Guru, who had been convicted of helping to plot a 2001 attack on the Indian parliament that left 10 people dead, had sparked protests in the disputed Muslim-majority region of Indian Kashmir.

India has made efforts to improve domestic security since the 2008 Mumbai attacks, in which 10 Islamist gunmen laid siege to the city, killing 166 people.

But experts say security forces still suffer from weak intelligence-gathering at the grass roots.

New Delhi blamed Pakistan-based militants for the Mumbai attacks, sending relations between the nuclear-armed neighbours into a deep freeze.

While Hindus form the majority of the population in Hyderabad, one of India's largest and most historic cities, there is a large community of Muslims living in the old quarter.