At least five people are missing, feared dead.

About 85 men were working in the tunnel when it caved in late on Monday night - it is thought because of floods.

Environmentalists oppose the controversial hydro-electric project at Tehri, saying the region is prone to earthquakes.

Tehri district official Puneet Kansal told the BBC that 50 people have been rescued. They included 15 injured in the incident.

India's federal electricity ministry has ordered an investigation into the tunnel collapse.

Officials said efforts were going on to clear a path into the tunnel.

"We are using heavy earth-moving machinery..to remove the rubble and earth," a local police official said.

Some of the survivors recounted the moment when the tunnel collapsed.

"It was a nightmare. I think I passed out amid the shouts for help, lack of air and falling earth. I just woke up outside gasping for air after someone pulled me out," K Shankar told the AFP news agency.

Correspondents say that the tunnel was being strengthened when boulders fell on it, causing it to collapse.

The Press Trust of India quoted local officials as saying the tunnel collapsed because of rising waters in the nearby Bhagirathi river.

Controversial project

The tunnel is part of a controversial hydro-electric power project which has seen massive protests from environmentalists for years, including the Booker Prize winning author, Arundhati Roy.

The floods have hit millions of people in the region

The 261 metre- (856 feet) high dam is the highest in Asia.

More than 100,000 people have been displaced by the 42-square-kilometre lake it has created - submerging the entire town of Tehri and nearly 125 villages.

This is the second such accident on the Tehri dam. Sixteen labourers were buried alive under piles of earth three years ago.

Despite the accident, work on the dam has not been suspended and officials plan to start generating electricity by next year.

In recent weeks parts of India, Nepal and Bangladesh have been hit by heavy monsoon floods leaving hundreds of people dead and millions homeless or stranded.