Police are searching a property in St Helens as part of a counter terrorism raid.
A 31-year-old has also been arrested on suspicion of being concerned in the commission, preparation or instigation of acts of terrorism.
He is currently in police custody.
A police spokesman said an address in View Road, Rainhill, was currently being searched by counter terrorism officers.
Raids are also taking place at a property in Kesington, Liverpool.
Police chiefs say the searches could last several hours or even days.
Det Chief Supt Tony Mole, from the North West Counter Terrorism Unit, said: “The NWCTU are currently conducting a joint investigation with Merseyside Police following the receipt of information from law enforcement agencies.
“It is our intention to carry out a thorough, professional investigation to determine the circumstances and details of the activity reported.
“We are taking all measures necessary to ensure public safety, which is our primary concern.
“Members of the public will see a lot of police activity at a number of addresses in the coming hours and possibly days.
“However, it is absolutely vital to stress – and I hope at the same time reassure the public – that there is no current threat or evidence of an imminent attack.
“We will of course keep the public informed as things progress and would once again encourage them not to be unduly worried.”
Anyone with concerns about suspicious activity should contact local officers or the confidential Anti-Terrorist Hotline on 0800 789321.