WASHINGTON (AFP) - The nearly 1.9 million-member Presbyterian Church USA has given its pastors the green light to marry gay and lesbian couples, in another coup for marriage equality in the United States.

The decision by the influential mainline Protestant denomination's General Assembly in Detroit, which ends on Saturday, follows "much thought, discussion and prayer", it said in a statement.

Its pastors will now be able to perform "any such marriage they believe the Holy Spirit calls them to perform" in those states - currently 19 out of 50 - where same-sex marriage is legal.

The biannual General Assembly also approved a change to the church's Book of Order, or Constitution, which will now define marriage as "traditionally" between a man and a woman.

The Presbyterians' positions stands in sharp contrast to that of the Roman Catholic church, the biggest-single denomination among Americans, and fundamentalist Evangelicals who strongly oppose gay marriage.

On Thursday, some 2,000 opponents of same-sex marriage staged a March for Marriage in Washington, despite opinion polls that indicate that a majority of Americans now accept marriage equality.

Three years, ago the Presbyterian Church USA voted to approve the ordination of openly gay and lesbian clergy.