Depravity and barbarism have made a sickeningly familiar return in Bombay. The co-ordinated viciousness, the targeting of a crowded station, popular restaurant and two hotels and the firebombing of a cultural monument have horrified an entire sub-continent. Terrorism has struck at the heart of the world's largest democracy.

The timing, tactics, orchestration and search for British, American and Jewish hostages point to an operation that has all the hallmarks of al-Qaeda. For weeks Western intelligence has given warnings, based on intercepts, that al-Qaeda was planning a spectacular atrocity before Barack Obama takes office. Typically, these jihadist extremists choose a soft target - Bali, Istanbul, or Bombay, also known as Mumbai - where security is lax and Westerners congregate. Random mass murder follows the capture or killing of senior al-Qaeda operatives. The aim is to demonstrate brutality that shocks and intimidates and leaves governments flailing for a response.

If this is a warning siren to President-elect Obama and a response to the targeting of al-Qaeda leaders in the tribal badlands of Pakistan, it is an even more deafening blast at India's fractious body politic. An unknown group calling itself the Deccan Mujahidin has claimed responsibility for these atrocities. Almost certainly this is a fictitious organisation. But India has already suffered a spate of bombings that have killed and wounded hundreds in its big cities. If the virus of fanaticism has taken root in India's Muslim minority, the future for a country built on tolerance, secularism and multi-ethnic balance looks grim.

With about 150 million Muslims, India is home to one of Islam's largest communities. Until recently, this minority appeared to have escaped the religious fundamentalism that has driven groups of Muslims elsewhere towards political extremism. But tensions have been rising. Communal violence always lies just beneath the surface, flaring up over the destruction by Hindu fanatics of the mosque at Ayodhya or during the anti-Muslim pogroms in Gujarat in 2002. And as Hindu nationalist groups have grown stronger, Muslims have felt more and more embattled.

If al-Qaeda, whose focus has been the destruction of Western power and the global dictatorship of a puritanical Islam, is now able to harness the communal resentments of millions in India, it will have won an ominous strategic victory. Statements issued yesterday were clearly intended to do just that. They spoke of vengeance for “stolen Muslim land”, they referred to the long-running sore of Kashmir and they called for the release of hundreds of Muslims imprisoned by the Indian State. Al-Qaeda has found the faultline in secular India, and is attempting, as it widens the fissure, to crack the foundations of Indian democracy and recent economic growth.

India's rapid advance has stalled, partly as a result of the global downturn, partly because bureaucracy and corruption have vitiated attempts to spread the benefits more widely. In the run-up to May's general elections, resentments are growing and minorities are being targeted. The political establishment lacks the cohesion, power and vision to confront the fundamentalists in each community. In desperation, it may seek scapegoats in Pakistan, blame Islamabad and jettison the recent rapprochement. This is a time for India's friends not only to share its grief, but to bolster its leaders' resolve and steady their response. The atrocity in Bombay threatens to drive a wedge between Hindus and Muslims in India; between India and its neighbour, Pakistan; and between the people of the sub-continent and the West. All of this would be to take revenge against precisely the wrong people for what is a terrorist act. The people to blame here, and the only people to blame, are the terrorists.