They're marching from the Shwedagon Pagoda to the country's largest city, Rangoon, protesting against the ruling junta.
A repressive military regime has ruled the country since 1962. It suppresses other political parties and organizations (gatherings of over five people can be against the law), has the judiciary in a stranglehold, and is notorious for sexual violence against enemies. Amnesty International has been concerned for a while.
These images of thousands of monks–people who have devoted their lives to reflection and meditation–marching peacefully for democracy as the Burmese military locks and loads, are nerve-racking and pretty powerful.