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Spanish authorities said Wednesday they had arrested two suspected militants in Madrid and found bullets and weapon magazines while conducting raids linked to the detentions.

According to the Europa Press news agency, the magazines were of the type used for assault rifles such as the AK-47.

The interior ministry said the two men were Spanish and had been detained for "glorifying terrorism," without giving any further details.

The arrests come as Spain strengthens security during the holiday season.

Authorities in Madrid are taking unprecedented measures for New Year's Eve, when thousands gather in the central Puerta del Sol square for traditional celebrations that see Spaniards swallow 12 grapes for each stroke of midnight.

All those entering the square will be searched.

Bollards will be used on nearby roads alongside police vans to guard against truck attacks like those that hit Nice in France in July, and Berlin on December 19.

Barcelona has taken similar measures.

According to Spanish authorities, 175 alleged jihadists have been detained since 2015, when Spain increased its terror alert to category four on a five-point scale.

Spain has been mentioned on extremist websites as a possible attack target for historical reasons, given much of its territory was under Muslim rule from 711 to 1492.

But unlike France or Belgium, the country is less exposed to the risk of return of radicalised nationals who went to fight abroad and plan to commit extremist acts on home soil.

Only around 200 Spaniards are estimated to have gone abroad to fight, compared to thousands from nearby France and Belgium.

In a separate incident Wednesday, Spanish authorities said that Turkey has arrested two Spanish women suspected of being part of the Islamic State group as they were leaving Syria accompanied by children.

Spain's interior ministry said the two women had been tracked for two years since they left for Syria from Ceuta, a Spanish enclave in northern Africa, accompanied by their husbands.

According to the statement, the husbands are "prominent" IS members and the wives, who were also part of the IS structure, are considered "extraordinarily valuable assets" for the organisation.

Madrid was hit by coordinated train bombings by an al-Qaeda inspired group in March 2004 that left 191 dead.

Agencies contributed to this report.