SYDNEY - Australia's highest court struck down a landmark law Thursday that had begun allowing the country's first gay marriages, shattering the dreams of more than two dozen same-sex newlyweds whose marriages will now be annulled less than a week after their weddings.

The federal government had challenged the validity of the Australian Capital Territory's law that had allowed gay marriages in the nation's capital and its surrounding area starting last Saturday.

The federal government's lawyer argued having different marriage laws in various states and territories would create confusion.

The High Court unanimously ruled that the ACT's law could not operate concurrently with the federal Marriage Act, which was amended in 2004 to define marriage as between a man and a woman.