Hundreds of people attended the opening on Thursday of the massive Ark Encounter Christian theme park in Williamstown, Kentucky, which features a full-scale replica of Noah' Ark.

"I believe this is going to be one of the greatest Christian outreaches of this era in history," said Ken Ham, president of Answers in Genesis and the builder of the ark. "People are going to come from all over the world."

While many Christians are praising the project, a group of atheists has warned more than 1,000 public school districts against taking their students to the theme park, saying it will violate the U.S. Constitution, the Associated Press reports.

The Freedom From Religion Foundation (FFRF), which campaigns for the separation of Church and state, is sending letters to public schools in Kentucky, Tennessee, Indiana, West Virginia and Ohio, telling them that they cannot organise student field trips to the Ark Encounter.

"This is a precautionary memo to advise that public schools and public school staff may not constitutionally organise trips to Ham's Ark Park, with its clear religious goal and portrayal of fiction as divine truth, or to the Creation Museum or any other religious venue," said the letter signed by FFRF co-presidents Annie Laurie Gaylor and Dan Barker.

They said schools have obligation to ensure programmes "do not inculcate religion."

FFRF claims that "taking public school students to a site whose self-professed goal is to convert children to a particular religion and undermine what is taught in public school science and history classrooms would be inappropriate."

"Public schools may not advance or promote religion," they said, adding that it would violate students' rights of conscience and the Constitution.

In building the theme park, Ham said, "We want to do as much as we can to reach everyone in the world with God's Word and the message of the gospel at the Ark Encounter. The Ark is a reminder to future generations of the truth of God's Word."

The ark was built based on biblical dimensions in Genesis 6, according to Answers in Genesis. It stands seven stories high and has a length of 510 feet, making it the largest free-standing timber-frame structure in the world.

A study estimated that between 1.4 million and 2.2 million people will visit the theme park in the first year including 40 percent or more who are non-Christians.