Nov 22 (Reuters) - Spain's anti-trust watchdog on Wednesday cleared Amazon, Booking Holdings and Tripadvisor of participating in or facilitating fake reviews on their websites.

CNMC, as the watchdog is known had been looking into a complaint lodged by a local association defending consumers.

"CNMC found no indication that the platforms have participated in or facilitated the publication of these false opinions," it said in a statement, adding that the companies invested in detecting fake reviews and collaborated with ongoing investigations.

The watchdog has, however, detected possible signs of a violation of consumer protection regulations, and therefore sent the complaint to the consumer rights authority.

No further details were given, however, the original complaint referred to possible indications that sellers and intermediaries contact users to publish false reviews in exchange for money or gifting them products, which falls within consumer protection regulations.

An Amazon spokesperson said his company is acting against fake reviews on its websites.

Booking Holdings and Tripadvisor did not immediately respond to messages seeking comment. (Reporting by Maria Luiza Amaral, editing by Inti Landauro and Elaine Hardcastle)