KABUL (Reuters) - A law curbing the spiraling costs of lavish Afghan weddings is proving difficult to enact, with many lawmakers opposed to legislation meant to contain crippling marriage bills in one of the world's poorest countries, a top government adviser said.

Since U.S.-backed Afghan forces ousted the austere Islamist Taliban in 2001, Afghans have revived the tradition of holding extravagant weddings, costing thousands of dollars, in a country where the average annual income is less than $400.

But after complaints from the families of grooms, who are expected to foot the bill and agree to every request of the bride and her family, the government has been working on new laws capping the number of guests at around 300 people.

"There was an idea in the Ministry of Justice to regularize that, to bring some kind of discipline, so that the family of the bridegroom does not suffer that much," said the ministry's top adviser Mohmmad Qasim Hashimzai.

"But then some people expressed opinions - especially some experts - that it was a private issue, that we should not intrude in the private business," he said.

Hundreds of guests attend Afghan weddings held in luxurious halls or in hotels, with clothing, food and music driving up costs with brides inviting the entire community to celebrations.

The government's bid to regulate the size of celebrations follows a ban last year on expensive weddings and dowries introduced by district governments in some areas to encourage young people to marry instead of postponing their nuptials due to spiraling costs.

The national government has since sought to introduce similar laws curbing expensive weddings across the country.

Parts of the law would also crack down on sleeveless dresses worn by women or other designs thought too revealing by religious conservatives.

Critics said the laws were a reversal for women's rights harking back to the Taliban period when weddings were monitored to ensure they did not breach hardline Islamic values including a ban on music.

Hashimzai said there had been significant opposition to the proposed law within President Hamid Karzai's cabinet.

"If the parliament approves it, it may go through. But there are lots of people who've expressed an opinion ... that it should not go through deeply in private matters, as weddings are a private issue," he said.

And practically, Hashimzai said, the laws would be near impossible to enforce even if they were introduced, as it would require the government to somehow attend every wedding and count guests, as well as judge costs.

"Do we need a policeman to go and stand over there?" he said. "I suppose some people may go to the family of the bride and say there is a rule and you cannot ask us to invite more than 300 guests. There will be some leverage. But I think it will be very difficult to control it."

(Reporting by Rob Taylor; Editing by Sanjeev Miglani)