After the series of bomb blasts that killed more than 45 people in two of India's main cities, the worry preoccupying all Indian politicians is whether Islamist extremism has finally taken root in India (see page 29).

Until now, India has been spared much of the extremist terrorism that has racked much of the Muslim world. Although it has one of the world's largest Muslim populations and has seen regular outbreaks of communal violence, India's Muslim minority has not been radicalised so far by the global jihadist movement. Despite three wars with Pakistan, terrorist infiltration and more than 60 years of tension, India's Muslims have not, on the whole, been seen as a fifth column under the sway of outside agitators. Al-Qaeda has no indigenous presence. India's secular constitution has been sufficiently robust to withstand assaults by religious extremists on all sides.

The bomb blasts in Ahmedabad and Bangalore, coming only two months after a series of explosions that killed 63 people in Jaipur, are therefore alarming. Admittedly, the devices themselves were crude, containing ball-bearings and other shrapnel and hidden in boxes and on bicycles. But there was a high degree of co-ordination, with some 17 explosions occurring almost simultaneously in Ahmedabad. The bombs were timed and placed to cause maximum casualties. And, for the first time, warnings were given and responsibility claimed by a little-known group calling itself the Indian Mujahideen.

The immediate suspicion is that outsiders have been attempting to stir communal riots by exploiting local grievances. No big city in India is immune from the threat of sectarian violence. But Ahmedabad is particularly volatile. Gujarat saw religious rioting in 2002 that left at least 1,000 dead, mostly Muslims. Narendra Modi, the Hindu nationalist chief minister, was accused widely of doing nothing to stop the attacks on Muslims, and his continued hold on power is resented deeply by the Muslim minority. Attacks on the other two cities, however, may be intended to strike at economic targets. Bangalore is the main centre of India's booming high technology industry. And Jaipur is an important centre for the country's swiftly growing tourist industry.

Bomb attacks are all too common in India, with trains, markets and religious sites being the main targets. Most, including the deadly explosions that killed more than 180 people on trains in Bombay in 2006, are blamed on Islamist militants acting with the help or under the orders of Pakistan's Inter-Service Intelligence Agency (ISI). There are again suspicions that shadowy figures in Pakistan or Bangladesh may have been involved in these latest attacks. If so, this shows the urgent need for the authorities in both these Muslim countries, which have a vital interest in better relations with their neighbour, to crack down on militants who thrive on tension to advance their own extremist agenda.

The Indian authorities have acted so far with sensible level-headedness. The Government has appealed for calm while criminal police and intelligence have begun investigations. Indian democracy is resilient and has absorbed greater shocks before. But any sign that the contagion of extremism has infected India's 120-million strong Muslim community should be treated with the utmost seriousness. All India could be at risk.