A Russian anti-gay group has asked prosecutors to investigate milk cartons that it claims promote homosexuality to children.

The label of the Vesyoly Molochnik milk, owned by multinational dairy company Pepsi Co, is adorned with a jolly milkman and a rainbow in the sky.

Anatoly Artukh from the People's Council, says the label is a violation of St Petersburg's anti-gay propaganda law.

"A rainbow appeared on the cartons, a world-renowned symbol of the gay movement," he said.

"That immediately put me on alert."

State prosecutors are now investigating if the label is attempting to promote homosexuality to youth, as Mr Artukh claims.

"I have no doubts about Pepsi Cola," Mr Artukh said.

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"This is a company renowned for actively and aggressively financing and promoting homosexuality."

For gay and lesbian groups, the issue is no laughing matter.

Polina Savchenko, the director of advocacy group Coming Out, says it is just another sign of the atmosphere in the city since the passage of the anti-gay propaganda law.

"Our life has been quite funny in the last year or so with the introduction of the homosexual propaganda law in November," she said.

"You know, it could be funny. It's so ridiculous, and so middle-aged, that it could be funny.

"But unfortunately it's not funny because we know that they're serious."

'People are afraid'

Ms Savchenko says the chill has well and truly set in.

"People are afraid. And this is actually the bottom line of what the effect, the grave effect of this law is," she said.

In fact, any promotion of gay equality has been targeted, including Madonna's appeal for gay rights at her recent concert in the city.

The pop star is now being sued for more than $10 million for inflicting moral damage.

"Her huge popularity might affect some young people who don't realise the danger of the gay movement," Mr Artukh said.

Whether it is Madonna or milk, for supporters of St Petersburg's anti-gay law, potentially illegal homosexual propaganda could be anywhere.