Indian Prime Minister Narendra Modi joined 50,000 students and other residents for a mass yoga session in the pouring rain in the northern city of Lucknow. Similar yoga displays were organized in villages, towns, and cities across India.
"Many countries which do not know our language, tradition, or culture are now connecting to India through yoga," Modi said in an address to the crowd.
"Yoga connects body, mind, and soul. It is playing a big role in bringing the world together too," he said after performing various poses.
The United Nations designated International Yoga Day in 2014. In Myanmar, people performed exercises near the Shwedagon pagoda, the landmark Buddhist stupa in Yangon.
The practice began in ancient India, and Modi has described yoga as free health insurance and exhorted people to make it a part of their daily lives.
Modi, dressed in white, instructed the children at the outdoor yoga session at Rama Bai Ambedkar Ground. Occasionally, he paused to correct a child's posture.
"It was a very nice gesture of prime minister to come out of the dry comfort of his waterproof tent and do yoga with us in the open. It was not easy. The yoga mat was wet and slippery and water puddles were all around," the 18-year-old student Neha Prakash said.
In the western city of Ahmedabad, 125,000 led by celebrity yoga guru Baba Ramdev gathered at an open-air ground to try to set a new Guinness World record for the largest session.
Police in New Delhi closed roads to make room for a mass yoga session held amid tight security in the heart of the capital.
Modi, who credits his strict yoga regime for his ability to work long hours on little sleep, has been spearheading an initiative to reclaim the practice as a historic part of Indian culture since his Hindu nationalist government came to power in 2014.
He has set up a ministry dedicated to promoting yoga and other traditional practices and persuaded the United Nations to create a dedicated International Yoga Day, a move seen as a triumph of soft power.
Indian scholars believe yoga dates back 5,000 years, based on archaeological evidence of poses found inscribed on stones and references to Yogic teachings in the ancient Hindu scriptures of the Vedas.
On Wednesday the UN headquarters in New York lit up with images of poses, among the events being held across more than 100 countries to mark the third International Yoga Day.
From China's Great Wall to the London Eye, yoga enthusiasts performed 'asanas', or poses, at major landmarks.
Across India, schoolchildren, soldiers, politicians, and bureaucrats bent and twisted their bodies on colorful mats at mass outdoor sessions.
Television footage showed Indian soldiers performing yoga in their military overalls in the Himalayan region of Ladakh, at a height of 5,500 meters.
Vishnudeo Vishwakarma, a retired former Air India employee, said he began every day with a 5 am yoga session.
"If you buy a machine and don't operate it... after one year, the machine will not run.
Your body is like that," the 66-year-old told AFP in Delhi. "Your joints and muscles will be stuck up."