Argentina earlier had used similar tactics against fishing fleets from Asian and Pacific nations that regularly operate in the Atlantic.
The Spanish Foreign Ministry Website had no comment until Friday on the reported interception of fishing vessels from Galicia, northwestern Spain, last month.
In Buenos Aires, official comments cited in the media said Argentina was within its rights to intercept all shipping to Falklands, which it claims is Argentine but under colonial British occupation.
Argentine forces attacked the islands in 1982 but were repulsed by Britain. The 74-day conflict was led by an Argentine military junta ruling the country at that time, which then had to sign surrender after British forces beat back the invasion.
About 1,000 civilian and military personnel died in the Falklands War, which boosted British Prime Minister Margaret Thatcher and hastened the departure of Argentina's military rulers.
A return to civilian rule in Buenos Aires didn't end Argentine claims on the Falklands, however.
Analysts said the Argentine action pitted Madrid against awkward choices and built up pressure on incoming Prime Minister Mariano Rajoy.
Falklands or Latin America wasn't on his mind when Rajoy declared after his November victory "there will be no enemies but unemployment, the deficit, excessive debt, economic stagnation and anything else that keeps our country in these critical circumstances."
Argentina's extension of the South Atlantic blockade of all Falklands-bound shipping means Rajoy is up against yet another angry group of Spaniards whose livelihood is at risk because of the Falklands factor.
About 40 fishing vessels and 600 crew members are likely affected by the Argentine measure, Spanish newspaper El Faro de Vigo said.
Spanish fishing fleets were told by Argentine patrol commanders their presence was illegal and in violation of Argentina's "legal" blockade of the sea channels to and from the Falklands.
El Faro de Vigo said the Spanish organization grouping fishing fleets operating on the high seas was warned of the action by the Argentine Embassy in Madrid.
The embassy said Spanish fishing vessels will be subjected to Argentine patrols because the "Malvinas, Georgias del Sur and Sandwich del Sur Islands and adjoining maritime spaces are integral part of the Argentine territory."
Las Malvinas is Argentina's name for the Falklands Islands. Argentina also insists on using its given names for all other British-ruled islands in the Falklands.
The embassy told the fishing body its members' vessels were operating illegally in the South Atlantic waters as they hadn't obtained licenses from the Argentine government. Argentine authorities, it said, felt duty bound "to put an end to all those illegal fishing activities."
The association argued its members shouldn't suffer the consequences of a dispute between Argentina and Britain.
There were no immediate reports of reaction from Madrid which, Argentine communications to the fishing association state, was informed of the patrols several times.