* Raids part of probe that began last year
* Spanish authorities say Cementos Portland, Cemex raided
* Holcim confirms offices searched, cooperating with EU
* Cemex confirms offices searched
* Holcim shares up, Lafarge, Heidelberg down
(Adds Cemex confirmation, details from European Commission, shares)
By Bate Felix
BRUSSELS, Sept 23 (Reuters) - Cartel investigators from the European Union executive raided a number of makers of cement and related products in Spain this week as part of a probe that began last year, the European Commission said on Wednesday.
Swiss firm Holcim confirmed it had been searched. Spanish authorities said the premises of Cementos Portland, an affiliate of builder FCC, and world No. 3 cement maker Cemex of Mexico had been raided.
The Commission, antitrust watchdog of the 27-country European Union, did not identify the companies involved. It said in a statement that it carried out the raids with the Spanish competition authority on Tuesday and Wednesday.
"The Commission has reason to believe that the companies concerned may have violated antitrust rules that prohibit cartels and restrictive business practices and or abuse of a dominant market position," the EU executive said.
A Commission spokesman said the raids were part of an investigation into the sector that began in November 2008.
Last year, cement giants Lafarge of France, Cemex, Holcim, Heidelberg Cement and Dyckerhoff of Germany said they had been targeted by the EU's investigation.
Holcim confirmed in a statement that its offices in Spain had been searched by the antitrust authorities and that it was cooperating with them.
Cemex also confirmed in a statement that its offices had been searched by the antitrust authorities.
Holcim shares were up 0.49 percent, Lafarge was 0.15 percent down, Heidelberg 0.99 percent lower and Dyckerhoff 1.3 percent off at 1534 GMT versus a 0.45 percent lower DJ Stoxx construction and building materials index.
The EU's antitrust watchdog carries out surprise raids as a preliminary step in investigating suspected anti-competitive practices.
If companies are found by the Commission to have breached European antitrust regulations, they could be fined up to 10 percent of their annual revenue. (Additional reporting by Katie Reid in Zurich and Judy MacInnes in Madrid; Editing by Jon Loades-Carter)