Abdul Halim is said to belong to the Ahle Hadees outfit, an ultra-conservative religious group that pledges its allegiance to the Wahhabi sect of Islam.

Several members of this group are also said to be part of the banned Students Islamic Movement of India, suspected of involvement in the blasts in Ahmedabad and Bangalore.

Halim was arrested on the basis of eyewitness accounts and a phone call traced to him, officials said.

'His arrest could well be the first in a series of more arrests likely to happen in connection with the serial blasts,' said Ahmedabad Police Commissioner Ashish Bhatia.

'We have got some definite leads and we are pursuing them but we cannot predict as of now when we will be able to crack this case.

'We have now picked up 10 to 12 people on various suspicions and they are being put through sustained questioning.'

Halim is said to have enticed disgruntled Muslim youths living in refugee camps in and around Ahmedabad, following the 2002 anti-Muslim violence in Ahmedabad and elsewhere in Gujarat state that left more than 2,500 people dead and property worth millions of rupees destroyed.

The police alleged that he was linked to an organised network that was engaged in sending the youths across the border to Pakistan for training in militant activities.

But the police said his arrest was in connection with Saturday's blast that claimed at least 49 lives. An Ahmedabad court yesterday ordered Halim to be held for 14 days.

Details are also emerging over a raid on a house on Sunday, where police seized a computer which an e-mail message claiming responsibility for the bombings had come from.

Police said the house was rented out to an American citizen working in Mumbai.

The 48-year-old American has not been detained, said Mr Kirit Sonawane, a police officer involved in the raid. Police provided no other details of the man's identity, saying only that 'we are talking to him'.

Police investigations are also concentrating on a couple of bicycle shop owners and distributors, as the bombs were planted on bicycles.

Following the serial blasts, security across major Indian cities has been further heightened.

In New Delhi, officials deployed 3,000 extra police officers to patrol markets, train stations and places of worship.

Security was stepped up in and around the Bombay Stock Exchange.

Prime Minister Manmohan Singh visited victims of the Ahmedabad blasts yesterday and asked political parties and investigative agencies to unite against terrorist attacks aimed at fomenting religious violence.

pjay@sph.com.sg