The civil trial, triggered last year by a classroom battle, marks the beginning of the first major legal assault on evolution science in 18 years. The case also represents the first legal test of "intelligent design", the belief that life on earth is too complex to be explained by random genetic mutation and therefore a guiding force must be involved.

In yesterday's court hearings, supporters argued "intelligent design" does not stipulate what that guiding force might be, and is therefore not a religion.

Its opponents derided it as a mere repackaging of creationism, the religious dogma that God brought life into being in its present form a few thousand years ago.

It is a test of strength which secularist organisations hope will prove decisive in destroying the scientific credibility of intelligent design once and for all. They are therefore determined to pursue it as far as the supreme court if necessary.

Witold Walczak, a lawyer for the American Civil Liberties Union (ACLU) arguing the case yesterday, told the Guardian before the trial: "It's the first vigorous review of intelligent design. They have so far refused to enter the forum where scientists publish their theories."

The contest was joined yesterday under the weak light bulbs of a federal district court in the Pennsylvania state capital of Harrisburg. In a chamber more accustomed to hearing arguments over taxes and copyright, lawyers debated the meaning of science and the origins of life.

The defendants were the school board from the school of Dover, Pennsylvania, which last year became the first district in the country to require its teachers to question the scientific underpinning of evolution.

"The theory is not a fact. Gaps in the theory exist for which there is no evidence," Dover teachers had to tell their students. "Intelligent design is an explanation of the origin of life that differs from Darwin's view."

The plaintiffs were 11 parents who claimed the statement was religious and therefore a violation of the constitutional separation between church and state.

Their legal team, backed by ACLU, launched an assault on intelligent design, describing it as a "clever, tactical repacking of creationism", which the supreme court ruled in 1987 could not be taught alongside evolution.

"It is a wedge strategy to overturn the rules of science," argued Eric Rothschild, the lead lawyer for the plaintiffs. "It's creationism with the words God and Bible left out. Intelligent design is not science in its infancy. It's not science at all."

The intelligent design case was argued by three lawyers from the Thomas More Legal Centre, a Christian foundation founded by Thomas Monaghan, a Roman Catholic multimillionaire and founder of the Domino's Pizza chain.

In his opening statement yesterday, Pat Gillan, the lead attorney for the defence, argued the case was "about freedom in education, not about a religious agenda".

Pointing out that the Dover statement asked school children to keep "an open mind", Mr Gillan said: "The primary effect of the policy would be to advance science education.

"It is not religion. Intelligent design is really science in its purest form - a refusal to close avenues of exploration in favour of a dominant theory."

In the US, the case is being portrayed as a replay of the Scopes trial of 1925, in which a Tennessee biology teacher was fined for breaking a state law banning the teaching of evolution. It was known as the "monkey trial" because the teacher, John Scopes, was derided for believing humans were descended from apes.

Secular science has won all the big legal battles since then, but not the struggle for American minds. In an echo of the Scopes trial, some of the Dover parents involved in the case were recently mocked at a local fair by opponents performing a monkey dance around them.

Opposing theories

Creationism
Literal interpretation of biblical Genesis. The earth was created about 6,000 years ago, with all forms of life in their current form

Intelligent design
Accepts the earth is millions of years old and that species can change. But living things show such complexity, a guiding hand must be at work

Evolution
Darwin's theory that modern species evolved from basic forms of life as a result of natural selection: random genetic mutation leading to species variation, with only variants adapted to the environment able to survive