(Reuters) - The Republican mayor of Indianapolis and a mainstream Christian group based in the city decried on Wednesday a bill Indiana's Republican governor plans to sign into law on Thursday that could allow companies to deny services to gays on religious grounds.

The bill was passed overwhelmingly in both houses of Indiana's Republican-controlled legislature and sent this week to the desk of Governor Mike Pence, who has said he will sign it.

Supporters of the bill say it will keep the government from forcing business owners to act in ways contrary to strongly held religious beliefs, while opponents say it is discriminatory.

Legal experts say the Indiana Religious Freedom Restoration Act sets a standard that will allow people of all faiths to bring religious freedom claims.

Gay rights groups worry it will be used by businesses that do not want to provide services for gay weddings, although those cases would probably end up being fought out in court.

Gay marriage became legal in Indiana last year following an appeals court ruling.

Whatever the legal intricacies, opponents say the law sends the wrong message.

"I don't believe this legislation truly represents our state or our capital city. Indianapolis strives to be a welcoming place that attracts businesses, conventions, visitors and residents," Mayor Greg Ballard, a Republican, said in a statement.

"We are a diverse city, and I want everyone who visits and lives in Indy to feel comfortable here. (The bill) sends the wrong signal," he said.

The Christian Church (Disciples of Christ), a mainstream Christian denomination headquartered in Indianapolis for almost a century, wrote a public letter to Pence on Wednesday saying the bill is "distressing" and urging him to veto it.

A day earlier Con Gen, a major gamers convention, threatened to stop holding an annual conference that draws tens of thousands to Indianapolis each summer, if the bill becomes law. Earlier, other big employers in Indiana argued that the bill could be economically damaging.

Republican Arizona Governor Jan Brewer vetoed a similar bill earlier this year due to concerns it could harm the economy.

(Reporting by Fiona Ortiz in Chicago; Editing by Eric Beech)