Three apparently co-ordinated attacks have killed at least 21 people and injured scores more between 6.50pm and 7pm in India's commercial capital, Mumbai.
Wednesday's attacks are the biggest since the 2008 assaults on Mumbai, which killed 166 people, which India blamed on Pakistan-based fighters.
Each blast took place in heavily congested areas during the financial capital's evening rush hour. Two explosions hit the south of the city and one hit the centre. Officials have speculated that the blasts, which occurred within minutes of each other, were organised by terrorist groups.
The first blast struck the Jhaveri Bazaar at 1854 local time, tearing through the well known southern jewellery market.
A minute later, a second blast hit the busy business district of Opera House, several kilometres away in southern Mumbai.
At 1905, the third bomb exploded in the crowded neighbourhood of Dadar in central Mumbai, according to police.
"People were shouting 'Help me, help me,'" another witness told Headlines Today television.
"It was just like mayhem everywhere," a witness told local news television NDTV.
NDTV reported that six people were killed in the Dadar blast and and 10 to 15 killed in total betwen the three attacks.
An NDTV reporter said he saw dead bodies being loaded into cars in the Opera House neighbourhood.
PM condemns attacks
Manmohan Singh, the prime minister, condemned the blasts and appealed to "the people of Mumbai to remain calm and show a united face".
Barack Obama, the US president, also condemned the attacks and offered support to bring the perpetrators to justice, according to a statement read by Jay Carney, the White House spokesperson.
"The US government continues to monitor the situation, including the safety and security of our citizens," the statement said.
Palaniappan Chidambaram, India's home minister, said 54 people had been admitted to hospitals with injuries.
Based on the apparent co-ordinated nature of the blasts, the home secretary said: "We infer that this was a coordinated attack by terrorists."
He also said that investigation teams from around the country were on their way to Mumbai.
Prerna Suri, Al Jazeera's India correspondent, said casualty numbers are expected to rise.
Speaking to a local CNN station, Jasraj Jain, a witness to one of the blasts, said: "We were inside our office when we heard a huge noise.
"Outside there was a lot of commotion, we can see fire trucks are here and they have taken away two or three bodies."
A guest appearing by phone on NDTV, said: "We're hearing from our sources that the government suspect the Indian mujahideen to be behind this."
The guest said that two members of the Indian mujahideen were arrested in Mumbai state on Tuesday.
The 2008 attacks on Mumbai were blamed on the Pakistani group Lashkar-e-Taiba (LeT) and strained ties between the two nuclear-armed neighbours.
The co-ordinated attacks targeted luxury hotels, a bus train station, a Jewish centre and tourist attractions popular with foreigners.
Source: Al Jazeera and agencies