Three bombs ripped through India\'s commercial capital Mumbai on Wednesday, killing 21 people and injuring more than 100 in the deadliest attack in the city since the 2008 assault by Islamist militants. The coordinated blasts hit busy districts in the south of the city, the same area targeted two and half years ago by Pakistan-based militants who caused mayhem and bloodshed during a 60-hour siege that left 166 people dead. \"It is another attack on the heart of India,\" said the chief minister of Maharashtra state, Prithviraj Chavan, who hinted at possible foreign involvement by saying it was a \"challenge to Indian sovereignty.\" The Home Ministry said at least 21 people had been killed and 141 injured, many of them seriously. Home Minister P. Chidambaram told reporters in New Delhi that it was \"a coordinated attack by terrorists,\" adding that \"the entire city of Mumbai has been put on high alert.\" The targets included a predominantly middle-class residential area, a wholesale gold market and a district housing diamond traders and jewellery shops, which is also home to Mumbai\'s abandoned opera house. All three explosions went off within a 15-minute period, starting at around 6:50pm (1320 GMT). Fashion photographer Rutavi Metha heard a \"massive\" explosion and ran towards the site of the bomb in the gold market area known as Zaveri Bazaar -- one of targets for explosions in 2003 that killed 52 and injured more than 150. \"There were bodies on the ground and a lot of blood. Local people were trying to help, picking up the injured and putting them in cars to drive them to hospital,\" she said. Indian Prime Minister Manmohan Singh condemned the blasts and appealed for calm. No group claimed responsibility, though suspicions initially fell on two Islamist groups that have targeted India in the past: the home-grown Indian Mujahideen and the Pakistan-based Lashkar-e-Taiba (LeT). US President Barack Obama was among the first to condemn the \"outrageous\" bombings and said the United States was closely monitoring the situation. \"The American people will stand with the Indian people in times of trial, and we will offer support to India\'s efforts to bring the perpetrators of these terrible crimes to justice,\" Obama said in a statement. In November 2008, 10 militants from the LeT attacked multiple targets in Mumbai, including five-star hotels, in an assault known as \"26/11\" in India and often compared to the September 11 attacks on the United States. India broke off a peace dialogue with Islamabad afterwards, blaming it for failing to crack down on the militant group. Talks between the two nuclear-armed rivals only resumed earlier this year. The Indian and Pakistani foreign ministers are scheduled to hold talks in New Delhi in the last week of July. Eyewitnesses outside the diamond trader building in south Mumbai said a car bomb had exploded when the area was packed with office workers heading home. Fire brigade floodlights showed the full extent of the damage in a claustrophobic side street in the Opera House area. Sandals were abandoned among the muddy puddles and potholes overflowing with steady monsoon rain; roadside food stalls were overturned in the gutter, and the mangled remains of a car stood with its doors hanging off. \"More than 5,000 people pass through here every minute,\" said Jayesh Labdhi, a committee member of the Mumbai Diamond Merchants\' Association near the wreckage of a white car. \"There was a huge big blast. We didn\'t know what it was but after a couple of minutes we saw so much smoke everywhere. There were so many people lying on the ground just crying out and wanting help,\" Labdhi told AFP. The last major bombing in India was in February last year in the western city of Pune, when a blast at a packed restaurant killed 16 people including several foreigners. In 2006, a series of seven high-powered blasts on suburban trains in Mumbai killed 187 commuters and left 800 injured -- an attack that India also blamed on Pakistan-based militants.