Here are 10 developments in the story:
PM Modi stayed for about two hours and called the event a "Kumbha Mela of culture". In an apparent message to criticism of the festival in the past few days, he said: "If we keep criticizing everything we have, the why should the world look at us?"
The three-day festival has been wrapped in controversy over allegations that irreparable damage has been done to the delicate ecosystem of the river by the organisers - Sri Sri's Art of Living foundation.
President Pranab Mukherjee had opted out of the event earlier this week. PM Modi's presence was viewed by many as a message to critics at a time the government faces allegations of going out of its way to facilitate Sri Sri's fest.
"Yes, this is a private party, because the entire world is my family," said Sri Sri, PM Modi seated near him.
Thousands came for the opening, unfazed by traffic or the heavy rain and hail that led to brief chaos at the 1000 acre area that includes a stage as big as six football fields for 35,000 performers from 150 countries including Pakistan, Brazil and Argentina.
The festival opened with chants and a traditional dance performed to music by 8,500 artistes.
Key roads in Delhi, mainly those near the festival venue were jammed. "Kindly avoid the stretch," the traffic police tweeted repeatedly.
Over the next three days, the festival will feature musical performances, mass meditation sessions and prayers led by Sanskit scholars. Sri Sri Ravi Shankar has described it as "a Cultural Olympics".
Environmentalists allege that the festival, spread over 1,000 acres of the Yamuna's floodplains, will irreversibly damage the area.
Earlier this week, the National Green Tribunal allowed the event but asked Art of Living to pay Rs 5 crore to restore the area. Today, the foundation promised to pay Rs 25 lakh first and the rest over three weeks.
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