"It's an amazing moment to be here and see such multilateral cooperation and so much hope," actor Sigourney Weaver said in New York as the signatures opened.
The treaty marks change in "the way we view the ocean, from a big garbage dump and a place where we can take stuff, to a place that we take care of, that we steward, we respect," she told AFP.
Sixty-seven countries signed the treaty on the first day, including the United States, China, Australia, Britain France, Germany and Mexico as well as the European Union as a whole, according to the U.N.
But each country must still ratify the treaty under its own domestic process. The treaty will come into force 120 days after 60 countries ratify it.
"It is clear that the ocean is in urgent need of protection," said Belgium's deputy prime minister, Vincent van Quickenborne.
Without action, "it's game over," he said.
After 15 years of discussion, the United Nations sealed the first treaty on the high seas in June by consensus, although Russia said it had reservations.
The start of signatures marks "a new chapter" of "establishing meaningful protections" for the oceans, said Nichola Clark of the Ocean Governance Project at The Pew Charitable Trusts.
The high seas are defined as the ocean area starting beyond countries' exclusive economic zones, or 200 nautical miles (370 kilometers) off coastlines — covering nearly half the planet.
Nonetheless, they have long been ignored in discussions on the environment.
A key tool in the treaty will be the ability to create protected marine areas in international waters— only around one percent of which are now protected by any sort of conservation measures.
The treaty is seen as crucial to an agreement to protect 30% of the world's oceans and lands by 2030, as agreed by governments in a separate historic accord on biodiversity reached in Montreal in December.
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