"Today, 13,000,000 tonnes of plastic leak into the oceans every year, what among other damage, kill 100,000 marine animals annually," the UN says.
World Oceans Day kicked off on Saturday as a reminder of the major role oceans have in everyday life.
This year, the UN General Assembly launched "Play It Out" — a global campaign against plastic pollution that includes a music festival.
"Today, 13,000,000 tonnes of plastic leak into the oceans every year, what among other damage, kill 100,000 marine animals annually," the UN said in a statement.
"While most plastics are expected to remain intact for decades or centuries after use, plastics in the oceans that do erode end up as micro-plastics that are consumed by fish and other wildlife, quickly making their way into the global food chain," it said.
"From plastic straws to plastic bags, we all are at the frontline of efforts to #BeatPlasticPollution."
The UN added that globally, the market value of marine and coastal resources and industries is estimated at $3 trillion per year, or about 5 percent of global GDP, showing the financial importance of oceans for the world.
More than 1,000 related events are being held around the world this year, including film screenings, clean-up activities, interactive talks and painting contests.
The UN General Assembly decided that as of 2009, June 8 would be designated as World Oceans Day to draw global attention to the benefits derived from the oceans and the challenges faced by the international community in connection with the oceans.
According to the UN, the concept of a "World Oceans Day" was first proposed in 1992 at the Earth Summit in Rio de Janeiro "as a way to celebrate our world's shared ocean and our personal connection to the sea".
'Dolphins are not entertainers'
On the occasion of World Oceans Day, World Animal Protection, an international non-profit animal welfare organisation, released a statement on its website concerning dolphins.
"Some dolphins are condemned to a life of utter boredom in the confines of a manmade concrete tank or artificial lagoon," it said, criticising "Swim with the Dolphins" programmes.
The movement also stressed that wild dolphins can swim more than 100 kilometres (62 miles) a day and can live for over 50 years, "which amounts to a life sentence in a small, chlorinated pool".
It also criticised the detention of dolphins in Australia for tourism.