Mexico's Supreme Court upheld a law allowing abortion in the nation's capital, in a victory for leftist city lawmakers over conservative President Felipe Calderon's government and the Catholic Church.

In an eight-three vote, judges said there were no grounds to overturn a law approved in 2007 by the Mexico City assembly that legalised abortions on demand during the first trimester and established free public clinics.

The landmark decision could open the door for other states in Mexico, the world's second most populous Catholic country, to follow the capital's lead in relaxing laws that criminalise abortion, legal experts said.

The judges found that Mexico's constitution did not explicitly guarantee the right to life of the fetus, and that the interests of the unborn had to be balanced with those of women seeking an abortion.

While President Calderon stayed largely on the sidelines of the battle, his attorney-general's office and the National Human Rights Commission sought to reintroduce a ban on abortion.