The 12 boys and their soccer coach rescued from a flooded cave in Thailand have waved, smiled and offered traditional "wai" greetings in their first public appearance at a national broadcast in the northern province of Chiang Rai.

Doctors, relatives and friends, some in yellow traditional garb, greeted the boys, aged 11 to 16, and their 25-year-old coach, who wore T-shirts emblazoned with a red graphic of a wild boar and carried in footballs they kicked gently on the set.

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"Bringing the Wild Boars Home," read a banner in Thai that greeted the soccer team on the set, designed to resemble a soccer field, complete with goalposts and nets, where the boys sat on a dais, beside five members of the rescue team.

But the moment was bittersweet, as two of the boys held up a framed pencil sketch of Samarn Kunan, 38, the former Thai navy diver who died while he worked underwater, laying oxygen tanks along a potential exit route out of the cave complex.

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"Everyone was very sad," said the coach, Ekkapol. "They felt like they were the reason he had to die and his family had to suffer."

"I told everyone fight on, don't despair," said one of the boys, recounting how they battled during the excruciating days spent in the cave.

Another, Adul Sam-on, 14, recalled the moment when two British divers found the group on July 2, squatting in a flooded chamber several kilometres within the cave complex.

"It was magical," he said. "I had to think a lot before I could answer their questions."

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That discovery triggered the rescue effort that brought them all to safety over the course of three days, organised by Thai navy SEALs and a global team of cave-diving experts.

The group had planned to explore the Tham Luang cave complex for about an hour after soccer practice on June 23. But a rainy season downpour flooded the tunnels, trapping them.

The group, which had eaten before going into the caves, took no food on the excursion, and had to subsist on water dripping from stalactites in the cave during their ordeal, he added.

"We only drank water," said one of the boys, nicknamed Tee.

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The team's youngest member, Titan, added, "I had no strength. I tried not to think about food so I didn't get more hungry."

The boys, who sported crisp haircuts, had gained 3kg each on average since the rescue, and ran through confidence-building exercises ahead of Wednesday's event, the hospital director said.

The rescue effort drew global media attention and hundreds of journalists, and excitement picking up again in the usually sleepy town of Chiang Rai ahead of the much-anticipated 45-minute live appearance broadcast on dozens of channels.

"We don't know what wounds the kids are carrying in their hearts," said justice ministry official Tawatchai Thaikaew, who asked for the boys' privacy to be respected after the discharge, for fear that media attention could affect their mental health.

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"The media know the children are in a difficult situation, they have overcome peril and if you ask risky questions then it could break the law," he told reporters.

King Maha Vajiralongkorn has granted permission for a party in the Royal Plaza, a public square in Bangkok's old town, to thank the Thai and foreign participants in the rescue, the government said.

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