Indian Prime Minister Narendra Modi performs yoga exercises along with over 35,000 people on Rajpath avenue, New Delhi on Sunday. Photo: IC

Prime Minister Narendra Modi on Sunday hailed the first International Yoga Day as heralding a "new era of peace," moments before he surprised thousands in New Delhi by taking to a mat himself to celebrate the ancient Indian practice.

Yoga-loving Modi led more than 35,000 people, including bureaucrats, students and soldiers, performing poses such as the half camel and cobra in a 35-minute mass outdoor yoga session on a New Delhi boulevard.

The event set two new Guinness Book of World Records - one for the largest yoga class at a single venue with 35,985 participants and another for the most number of nationalities to attend with people from 84 countries, a government statement said late Sunday.

The previous record was set in 2005 when 29,973 students in the central Indian city of Gwalior gathered for a yoga class.

"Yoga is more than only physical fitness. We are not only celebrating a day but we are training the human mind to begin a new era of peace," Modi told the crowd along Rajpath avenue.

He thanked the United Nations and the 177 co-sponsoring countries for adopting his idea for a world Yoga Day.

"This is a program for human welfare, a tension-free world and a program to spread the message of love, peace and goodwill," said the premier, wearing a crisp white outfit with a national tricolor scarf.

Modi then surprised the crowds by leaving the stage, removing his glasses, and securing a spot at the front of the massive session to mirror the stretches, breathing, and meditative moves beamed on giant screens along the historic avenue.

The prime minister, who credits yoga for his ability to work long hours on little sleep, had been scheduled only to make a speech at Rajpath, where colorful mats were lined across the stretch that connects the presidential palace with the iconic India Gate monument.

In his maiden address to the United Nations General Assembly, Modi proposed to dedicate a day devoted to the ancient Indian discipline, prompting the UN to proclaim June 21 as the International Day of Yoga.

But the government's push for "yoga for harmony and peace" met with criticism in the run-up to Sunday, with some religious minorities accusing Modi of pushing a pro-Hindu agenda in officially secular India.

Foreign Minister Sushma Swaraj has repeatedly emphasized that the craft has no link to any religion, describing it as "the science of integrating body, mind and soul."

"At a time when ethnic conflicts and extremist violence are threatening to destabilize societies, yoga can serve as the perfect antidote to ... move us on the path of harmony and peace," Swaraj said at the UN's Yoga Day launch in New York, where scores struck a pose in Times Square.