British police have arrested four men suspected of preparing terrorists acts.

Police said Friday the arrests and searches of several properties in west London and the Thames Valley are part of an ongoing investigation into "Islamist related terrorism."

Police said armed officers participated in two of the arrests but that no shots were fired.

The men range in age from 19 to 27. Searches are continuing.

Officials have classified the threat level in Britain as "severe," meaning a terrorist attack is viewed as highly likely.

There have been numerous arrests in recent weeks as police have taken action against some Britons who have returned from fighting in Syria and are feared to be planning attacks in Britain.

Police have been warned to be vigilant against attacks on officers.