Four men were being questioned today under the Terrorism Act.
South Yorkshire Police said the men were arrested in the Sheffield area on Tuesday after three addresses were raided.
A spokesman said items of property were also recovered.
South Yorkshire Police and officers from the Anti-Terrorist Branch executed search warrants under the Terrorism Act 2000.
A spokesman said: "The men are still being questioned by police who on Wednesday were granted a further 72 hours to question them by a district judge at Sheffield Magistrates Court."
He added: "No information is available to say that there is any risk to the public regarding these arrests.
"Although no details of the property are being released, we can say no hazardous substances, weapons or ammunition were recovered."
The inquiry is believed to involve international terrorism.