Rescue chief said first boy was expected to be brought out of cave by around 9:00 pm (1400 GMT), meaning trip would take around 11 hours.
Mae Sai: Elite divers on Sunday began the extremely dangerous operation to extract 12 boys and their football coach who have been trapped in a flooded cave complex in northern Thailand for more than two weeks, as looming monsoon rains threatened the rescue effort.
The "Wild Boars" team has been stuck in a cramped chamber several kilometres (miles) inside the Tham Luang cave complex since June 23, when they went in after football practice and were hemmed in by rising waters.
Their plight has transfixed Thailand and the rest of the world, as authorities have struggled to devise a plan to get the boys and their coach out through twisting, narrow and jagged passageways that in some places are completely flooded.
"Today is the D-day. The boys are ready to face any challenges," rescue chief Narongsak Osottanakorn told reporters near the cave site as weather forecasters warned of more monsoon rains late on Sunday that would cause more flooding in the cave.
Narongsak said the first boy was expected to be brought out of the cave by around 9:00 pm (1400 GMT), meaning the trip would take around 11 hours.
The boys, aged from 11 to 16, and their 25-year-old coach were found dishevelled and hungry by British cave diving specialists nine days after they ventured in.
But initial euphoria over finding the boys alive quickly turned into deep anxiety as rescuers raced to find a way to get them out, with Narongsak at one point dubbing the effort "Mission Impossible".
The death of a former Thai Navy Seal diver who ran out of oxygen in the cave on Friday underscored the danger of the journey even for adept professionals.
Saman Kunan had been trying to establish an air line in a flooded area with oxygen tanks when he passed out and perished.
After a short deluge of rain on Saturday night and with more bad weather forecast, Narongsak on Sunday said authorities had to act immediately.
"There is no other day that we are more ready than today," he said. "Otherwise we will lose the opportunity."
Between the operating base manned by Thai Navy Seals inside the cave and the trapped boys are twisting, turning cave passageways with torrents of water gushing through.
The water in the cave is muddy and unclear, with one diver comparing it to a cafe latte. Ropes have been installed to help guide the boys through the darkness.
Narongsak said Sunday two divers would accompany each of the boys out of the cave.
Rescuers had fed a kilometres-long air pipe into the cave to restore oxygen levels in the chamber where the team was sheltering with medics and divers.
More than 100 exploratory holes had also been bored -- some shallow, but the longest 400 metres deep -- into the mountainside in an attempt to open a second evacuation route and avoid forcing the boys into the dangerous dive.
On Saturday, Thai Navy SEALS published touching notes scrawled by the trapped footballers to their families, who had been waiting for them agonisingly close by outside the cave entrance.
The boys urged relatives "not to worry" and asked for their favourite food once they were safely evacuated, in notes handed to divers.
In one, Pheerapat, nicknamed "Night", whose 16th birthday the group were celebrating in the cave when they became stuck on June 23, said: "I love you, Dad, Mum and my sister. You don't need to be worried about me."
The coach, Ekkapol Chantawong, who many Thais have criticised for leading the boys into the cave, also apologised.
"To all the parents, all the kids are still fine. I promise to take the very best care of the kids," he said in a note given to divers on Friday.
"Thank you for all the moral support and I apologise to the parents."