The NGT earlier this month had asked the West Bengal government to prevent any violation of its 2017 order banning any Chhath ritual or other festivals in the lake known for its rich bio-diversity.
The Kolkata Metropolitan Development Authority (KMDA), which is responsible for the upkeep of Rabindra Sarobar, said thousands forced their way inside by breaking the locks, and claimed it was monitoring the developments.
"There was an order to keep all the 15 gates of the Rabindra Sarobar locked from Saturday to 11 a.m. to Sunday. But the people broke open the locks and entered by pushing us aside. First a handful of people entered, and then as the day progressed several thousand people were there.
"We couldn't do anything given the size of the mob through the day," said a private security official posted at Rabindra Sarobar.
The crowd also removed banners proclaiming ban on entry and shouted slogans before the police personnel, who virtually disappeared.
Later in the day, around 100 cops were deployed, but they also pleaded their helplessness as the crowd inside jived to songs played by a DJ, burst crackers and performed the puja.
West Bengal Minister Partha Chatterjee ducked questions about the incident, saying it was a matter to be looked into by the KMDA.
Enviromentalist Subhas Dutta rued the day's happening, saying the NGT order was aimed at preserving the lake, vital for maintaining the ecosystem of the area.
Devotees claimed they had been performing Chhath at Rabindra Sarobar for decades, and had no intention to conduct the rituals at any other location.
( With inputs from IANS )