Spanish police arrested four people on Saturday in its North African enclave of Ceuta suspected of belonging to a militant Islamist network, the interior ministry said.
Spain has stepped up security as well as efforts to prevent the radicalisation of young Muslim citizens following attacks in Paris this month in which Islamist gunmen killed 17 people.
Police are investigating whether the four people detained were planning an attack in Spain, the ministry said in a statement. It gave no other details and could not immediately be reached for further comment.
Sources close to the investigation told El Pais newspaper the inquiry started two weeks ago when one of the four posted a video on his Facebook page showing him providing military training and urging others to wage jihad.
Spain is among a number of European countries struggling to deter young Muslim citizens from becoming jihadists in Syria or Iraq, fearing they might return to plot attacks on home soil.
The Spanish cabinet has said it will put forward a plan to counter radicalisation among Muslim citizens at its weekly meeting next Friday.
Spanish and Moroccan police arrested seven people in December in a joint operation to prevent the recruitment of women to go to Syria and Iraq to support Islamic State insurgents there.
Last September, Spanish police arrested nine people suspected of belonging to a militant cell linked to Islamic State in Melilla, another Spanish enclave on the northern coast of Africa.