Justin Bieber -- Building School in Guatemala ... Without Shirt

Justin Bieber ventured deep into the Guatemalan jungle to help a group of poor villagers construct a badly needed schoolhouse this weekend ... and TMZ has the pics.

After Justin performed in Guatemala City on Saturday, he joined up with Pencils of Promise -- an organization dedicated to building schools for communities in need.

JB was choppered into a remote area outside the city on Sunday ... where a makeshift construction crew was already busy putting up walls for the new school.

In a video posted on his blog, Bieber says the town was overrun with poverty -- "I'm not talking about poor, I'm talking about 10 people living in a little shack."

Inspired, Justin ditched his shirt and went to work -- grabbing a wheelbarrow and helping to build a wall for the school.

He also took off his shoes and gave them to one of the local children.

Afterward, Bieber says it was, "one of the most amazing powerful experiences of my life."

Bieber says the experience helped him grow as a person -- saying, "I feel like they helped me as much as I helped them."