Hundreds of people have been mobilized to find the youngsters who went into the Tham Luang cave on Saturday and were trapped when heavy rains flooded its main entrance.
Anxious relatives camped out to perform traditional rituals, making offerings and reciting emotional prayers for their children's safe return.
"I asked for all God's wishes, but I'm certain in my heart that they will survive. They have been inside the cave before," the father of one of the young soccer players said.
Some relatives wailed at the cave's entrance near the Laos and Myanmar border, where huge crowds have gathered near stockpiles of water and food.
"My child, I'm here to get you now," one crying parent said, while another screamed: "Come home my child!"
The children, aged between 11 and 16, are thought to have retreated further into the tunnel as monsoon rains fell and flooded the cave, believed to be several kilometers long.
Rescuers found bicycles, football boots and backpacks at the entrance to the site Monday, and divers said they spotted footprints in one of the cave's chambers.
Park officials, police and soldiers were dispatched on foot, while an aerial team was also being mobilized to see if there was another entrance to the cave, according to an AFP photographer at the scene.
Navy divers equipped with oxygen tanks and food rations entered the cave early Tuesday in northern Chiang Rai province where rain continued to fall.
Rescue teams were also dispatched to light the inside of the cave and drain water from the site and an underwater robot was set to be dispatched to survey the area and help rescuers.
Interior Minister Anupong Paojinda said they were working around the clock to find the youngsters.
"We're working against time, but we're not panicking," he told reporters.