A fairy tale in which a mute cleaner in a government laboratory falls in love with an aquatic creature facing dissection there - an appeal to embrace differences - premiered at the Venice film festival on Thursday.

Mexican director Guillermo del Toro's "The Shape of Water" stars Sally Hawkins as Elisa, the cleaner in the lab whose life changes when she befriends the amphibious creature, which has been captured by government scientists to study its breathing patterns for use in advances in space travel.

"It's a very political, very fabulistic fairy tale about love and coming together..., about how we are told to stay apart for stupid reasons when we are all together," del Toro, a 52-year-old, previous Academy Award nominee, told Reuters.

The film is one of 21 U.S. and international movies vying for the Golden Lion that will be awarded on Sept. 9 after days of screenings, parties and red carpet glamour.

"The Shape of Water" features a fantastical creature - a cross between human and fish with glowing spots on its skin - to point to "the otherness" people so often reject, Del Toro said.

But the notion of otherness is evoked through other characters as well, be it Elisa's black friend Zelda, played by Octavia Spencer, or her secretly gay neighbour, played by Richard Jenkins.

"I’m Mexican and I know what it is to be looked at as the 'other'," Del Toro said. "The creature represents something that can be either divine or debased according to who looks at it."

Michael Shannon stars in the movie as Strickland, a ruthless government agent uninterested in genuine science and willing to cut the creature into pieces, if that is what he is ordered to do.

"He doesn't see anyone because his arrogance is so big," Del Toro said. "It speaks about the issue we have today that choosing fear over love is a disaster."

Hawkins was told about the role at a time when she herself was writing notes for a film about a mermaid, a coincidence she said was "magic and rare".

"When all those synchronistic, odd things that are almost beyond you happen, then you know it’s for a good reason," the Britain-born actress said.

The film is set in 1960s America but Del Toro, the maker of movies such as "Hellboy" and "Pan's Labyrinth", said the tale is relevant today.

"It's so hard to talk about emotions these days ... and talk about love but it's still the strongest force in the universe. The Beatles and Jesus cannot be wrong."