At least 20 people were killed and more than 50 wounded when bombs ripped through a crowded suburb of the Indian city of Hyderabad on Thursday in what police called an act of terrorism.
Prime Minister Manmohan Singh vowed punishment of those responsible for the "dastardly act", which came with the nation on alert after the recent hanging of a separatist unleashed protests in the Muslim-majority region of Kashmir.
The bombs targeted a mainly Hindu district in Hyderabad, a southern hub of India's computing industry which has a large Muslim population. The Australian and Indian cricket teams are due to play there in a match starting on March 2.
Doctors struggled to treat a stream of wounded victims as bloodied patients lay on stretchers at nearby hospitals and anguished relatives clamoured for news of their loved ones, an AFP photographer saw.
At least two devices went off in the evening explosions, and bomb-disposal experts were trying to defuse three more unexploded devices, police said.
"This is a terror attack for sure," Hyderabad deputy inspector of police Shiv Kumar told AFP, while adding there had been no claim of responsibility.
Twenty people died in the attack and 54 people were wounded, 35 seriously, he said.
"This is a dastardly act and the guilty will not go unpunished," Prime Minister Singh said of the attacks, the most serious to hit India since 13 people died in a 2011 bombing outside the High Court in the capital New Delhi.
But Singh, speaking in New Delhi, also appealed for "calm" in the aftermath of the Hyderabad blasts.
Foreign Secretary Ranjan Mathai, the top civil servant in India's external affairs ministry, said the culprits were not known but did not rule out foreign involvement.
"I am not sure there is any evidence it could be homegrown terrorism. We have had a number of attacks which have been traced to inspiration or leadership outside the country," he said at a Washington think-tank.
"I think we have to wait until the investigation reports have been completed," added Mathai.
The foreign secretary avoided direct criticism of nuclear-armed rival Pakistan, which New Delhi blamed for the 2008 Mumbai attacks in which 166 people were killed when Pakistani-born gunmen laid siege to the city.
Hyderabad 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.
Large crowds gathered near the site of the explosions in the Hyderabad suburb of Dilsukh Nagar as police struggled to collect evidence. The charred wreckage of parked scooters lay on the ground.
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.
In May 2007, at least 11 people were killed in a blast at a mosque in Hyderabad and five more died when police fired at Muslim protesters.
Months later in August, at least 40 people were killed in Hyderabad when two blasts hit an auditorium and an outdoor restaurant.
It remains unclear who carried out the 2007 attacks.
The latest explosions 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, has driven up tensions in the Himalayan region of Kashmir, which India disputes with Pakistan.