By Gulsen Solaker and Tuvan Gumrukcu

ANKARA (Reuters) - Turkey's parliament approved a law to allow some state-employed religious officials to administer civil marriages, a move opposition parties view as another blow to secularism and women's rights under President Tayyip Erdogan.

The law, passed late on Thursday, will allow muftis - officials employed by the state's Religious Affairs Directorate - to perform civil marriages that were previously administered only by municipal officials.

The secularist main opposition Republican Peoples Party (CHP) has vowed to take the law to the Constitutional Court "as soon as possible", while the leftist pro-Kurdish Peoples' Democratic Party (HDP) has also opposed it.

Critics fear the new law could lead to an increase in underage marriages, citing the religious ceremonies practiced in rural areas where brides are sometimes under 18.

"This paves the way for child brides because according to Islam, there is no age limit, a girl who has reached puberty can get married," CHP Deputy Chairman Ozgur Ozel told Reuters.

Ozel said the law included "constitutional breaches" and disregarded the founding values of the constitutionally secular Turkish republic.

Civil marriage under the age of 18 is illegal in Turkey, although in rural parts of the Sunni Muslim nation of 80 million religious ceremonies are common.

HDP lawmaker Huda Kaya said the law was a "slightly formalized" version of a previous controversial proposal that critics said could allow men accused of sexually abusing girls to avoid punishment if they marry their victims. The proposal was retracted from parliament last year after a public uproar.

'AGAINST SECULARISM'

"They say this is against secularism. The church does this in the West," Erdogan said last week.

Erdogan, whose roots are in political Islam, and his ruling AK Party say the law is designed to speed up the officiating of weddings, normally carried out by mayors at municipal offices.

But one CHP lawmaker, Ali Seker, said each state official had on average fewer than one wedding to conduct per day last year, citing official statistics during a debate in parliament.

The law states that the offices of muftis will be given the right to carry out weddings, meaning the religious officials could also appoint Muslim clerics or imams to officiate civil weddings - an authority that has never been granted to non-state employees in modern Turkey.

"This is the act of legalising a social event that is part of Turkey's traditions. The spirit of the laws must be in harmony with the spirit of social life," Justice Minister Abdulhamit Gul said.

Erdogan has spent his political career fighting to bring religion back into public life in Turkey and has cast himself as the liberator of millions of pious Turks whose rights and welfare were neglected under the secular elite.

Liberal Turks see Erdogan as attempting to roll back the work of Mustafa Kemal Ataturk, the Western-facing founder of modern Turkey.

"This is a clear alternative to secularism. Even bringing this bill on the agenda is igniting a spark against secularism," said Canan Gullu, chairwoman of the Turkish Women's Associations Federation, told Reuters.

"No matter what they do, women's fight will continue, and democracy and secularism will survive."

(Writing by Tuvan Gumrukcu; Editing by David Dolan)