More than 400 young people yesterday forgot their Monday blues by going green instead.

Spread out over rustic Pulau Ubin and Coney Island - offshore islands north-east of Singapore - they picked up litter, pulled up weeds, planted trees and built fences to catch sea debris.

It was part of Outward Bound Singapore's (OBS) first outdoor conservation classroom, which aims to help people better appreciate the natural environment.

"It is much easier to litter than to clean up," said Darren Ng, 15, after pulling out a fishing net buried deep in the sand with the help of three friends.

"I am more conscious about taking care of the environment and not littering," added the Secondary 3 student from St Gabriel's Secondary School.

In all, the volunteers picked up enough trash, ranging from plastic bags to discarded tyres, to fill 300 bags. The rubbish collected weighed a total of 2.4 tonnes.

The volunteers, mostly aged between 15 and 35, comprised mainly alumni from past OBS programmes, although some roped in friends and family.

Senior Minister of State for National Development Desmond Lee was the guest of honour at the event. He told the media that the event was about "getting Singaporeans into the thick of action".

"It is a call to action, and they have answered it... out of their own time; some took leave, some came from school, others during the holidays," said Mr Lee.

Besides picking up trash and pulling up weeds, the volunteers also planted 60 saplings. OBS deputy director of programmes and partnerships Ng Thian Choon said there are plans to make the outdoor conservation classroom a yearly event.

He said: "We go outdoors often and use it for our leisure and our own purposes... I hope participants are reminded of their responsibility to protect, conserve, beautify and improve it."