Buddhist monks and laymen gathered at temples throughout Thailand Thursday to pray for Thai King Bhumibol Adulyadej, 78, who was admitted earlier in the day to a Bangkok hospital to undergo an operation on his spine.
Thousands of Buddhist temples nationwide staged hour-long prayers for the revered king, who was scheduled for back surgery Thursday night.
More than 1,000 well-wishers had gathered outside Siriraj Hospital to welcome King Bhumibol, as he checked in to the hospital Thursday afternoon.
Yellow is the king's colour because he was born on a Monday, and each day of the week is associated with a specific lucky colour, according to Thai beliefs.
Thailand last month staged celebrations to mark King Bhumibol's 60th year on the throne, making him the world's longest reigning monarch.
Donning yellow shirts has become the fashion statement of the year, symbolising devotion to their monarch.
Doctors earlier this week revealed that the king had been diagnosed in 2003 as suffering from lumbar spine stenosis, a narrowing of the canal that contains the spinal cord and nerve roots, which is caused by ageing.
Doctors initially recommended physical therapy, but on May 3, a team of specialists resolved to perform "a microsurgical decompression" to relieve the condition, which causes back and leg pain and numbness in the legs when standing and walking.
The king, who has undergone a heart procedure in the past, reportedly cracked a rib on June 24 while performing walking exercises.