More than 250 officers, some wearing chemical, biological and radiological protection suits, shot one man and arrested another during a dawn raid on an east London house on Friday.

Police made no official comment on the reports but said nothing suspicious had been found in an initial search of the property. They had also reassured the public in the surrounding area there was nothing to suggest they were at risk.

The operation, one of the biggest since last July's suicide bombings in the capital, was prompted by suspicions that the house could have been used for making bombs or chemical weapons.

"Because of the very specific nature of the intelligence, we planned an operation that was designed to mitigate any threat to the public either from firearms or from hazardous substances," said Peter Clarke, head of the UK's anti-terrorism branch.

Some newspapers, citing unnamed security sources, said police believed suspected militants had made a "dirty" chemical device -- a conventional bomb surrounded by toxic material that could be set off by a bomber wearing a suicide jacket.

"We are absolutely certain this device exists and could be used either by a suicide bomber or in a remote-controlled explosion," one source told the Sun newspaper.

Newspapers quoted security chiefs who they said believed an attack was imminent, with possible targets including the underground train network or pubs crowded with fans watching the soccer World Cup tournament, which starts next week.

"We're 100 percent certain that an attack was being planned. If we haven't stopped it, it could take place very soon," the Daily Mirror quoted a police source as saying.

The cordon thrown around the house was also much smaller than that used to guard a property used as a bomb-making factory by four British Islamists who carried out last July's attacks on London's transport system which killed 52 commuters.

Detectives said the latest operation was not linked to those attacks on three underground trains and a bus although the country has been on high alert since then.

London police Commissioner Ian Blair, Britain's top officer, has said three terrorism plots have been thwarted since the July bombings and groups were planning further attacks.

The 23-year-old man, shot at the house during the raid is now recovering in hospital. His injury is not said to be life-threatening and he has been arrested on suspicion of "the commission, preparation and instigation of acts of terrorism."

A second man was also arrested at the house under the Terrorism Act.

Neighbours said the family who lived at the house were Bangladeshi, describing them as friendly and "very religious."