India's major cities have been put on a state of high alert after two blasts shook the southern city of Hyderabad, killing at least a dozen people and injuring many more.

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Security services were still trying to uncover responsibility for the explosions which occurred 10 minutes apart from one another, in attacks that the Indian Prime Minister Manmohan Singh labelled as "dastardly".

Federal Home Minister Sushilkumar Shinde said the bombs were attached to two bicycles about 150 meters (500 feet) apart in Dilsukhnagar, a residential and commercial district in Hyderabad, Southern India. Eight people are thought to have died in the first explosion and three in the other, which occured shortly after 7pm.

There were at least 50 injuries, with scores of people sent to local hospital, police said. The blasts occurred about two minutes apart outside a movie theater and a bus station.

Appealing to the public to remain calm, Mr Singh said: "This is a dastardly attack, the guilty will not go unpunished."

Hugo Swire, MP for Devon who is an official responsible for Britain's relationship with India, said he was "appalled" by the attacks.

"Having just spent three days with the Prime Minister seeing the optimism and opportunities across India, these attacks are a shocking reminder of the terrorist threat this country faces.

"I condemn this cowardly act of terror and, as agreed between the Prime Minister and Prime Minister Singh, the UK will continue to work closely with India to tackle the threat of terrorism wherever it occurs."

No one has yet claimed responsibility for the attacks. Early reports indicated that Indian security services had not picked up any alerts of a possible attack in Hyderabad.

Security has since been stepped up in sensitive areas, including railways and bus stations in major cities across India. A senior Delhi police official said security personnel have been alerted and they were keeping a tight vigil on the city, which David Cameron visited this week.

The last major bomb attack in India was in September 2011 blast outside the high court in New Delhi that killed 13 people.