British detectives have been given a further seven days to question four men arrested in connection with an alleged Islamist terror plot.

The men, aged from 19 to 27, were arrested overnight Thursday on suspicion of being involved in the commission, preparation or instigation of acts of terror.

Media reports said the alleged plot was planned for British soil.

The four suspects were arrested in west London and west of the capital. Armed officers were present at three of the arrests but no shots were fired, police said.

Several addresses have been searched.

"Warrants for further detention have been granted for all four men for a further seven days," said a spokesman for the Scotland Yard police headquarters.

All four men remain in police custody.

"These arrests and searches are part of an ongoing investigation into Islamist-related terrorism," Scotland Yard said.

The arrests come a few months after the national terror threat level in Britain was raised from substantial to severe, meaning a terrorist attack is considered "highly likely."

Police tape could be seen across the front of two residential addresses in High Wycombe, west of London, including one with several satellite dishes attached, with officers on duty outside.