"No to war," chanted many among some 6,000 demonstrators on Saturday according to march organizers as they rallied outside the Reina Sofia museum in the Spanish capital.
Protesters answered a "not in our name" petition from dozens of artists demanding Spain does not become embroiled in the Syrian conflict, AFP reported.
The petition has received some 34,000 online signatures to date as European governments consider armed intervention against what they say is ISIL following the Nov. 13 Paris attacks for which it claimed responsibility.
With Dec. 20 polls fast approaching Rajoy's conservative government has been holding off on any decision.
"Decisions have to be well thought though, as in any aspect of life," said Rajoy, who added Madrid was in touch with its allies pending a clear plan of action.
Rajoy's Popular Party is mindful of how in March 2004 under his predecessor Jose Maria Aznar, who had backed the US invasion of Iraq a year earlier, lost general elections which took place three days after extremists killed 191 people in a series of Madrid train bombings.
Aznar's stance on Iraq was in stark contrast to that of the public in a traditionally pacifist country.
Leftist opposition parties have voiced opposition to Spanish military involvement in the region while the leader of far-left grouping Podemos, Pablo Iglesias, backs putting the issue to a referendum.