The 4.5km tunnel is being constructed between Silkyara and Dandalgaon to connect two of the holiest Hindu shrines of Uttarkashi and Yamnotri.
Photographs released by the government rescue teams showed huge piles of concrete blocking the wide tunnel, with twisted metal bars on its broken roof poking down in front of the rubble.
“Pray to god that those workers trapped inside the tunnel are brought out safely,” Uttarakhand chief minister Pushkar Singh Dhami wrote on social media platform X.
A local police officer told the Press Trust of India news agency they were “very optimistic” the men would be rescued safely, but added it was “difficult to say how long it will take”.
The tunnel is part of Prime Minister Narendra Modi’s favourite Char Dham Road Project meant to improve connectivity for some of the most popular Hindu shrines in the country, as well as areas bordering China.
Accidents on large infrastructure construction sites are common in India.
In January, at least 200 people were killed in flash floods in ecologically fragile Uttarakhand in a disaster that experts partly blamed on excessive development.