BANGKOK — Myanmar began its largest crackdown on protesters since the civilian government of President Thein Sein came to power 20 months ago, mounting a raid on Thursday on hundreds of Buddhist monks and villagers opposed to the expansion of a Chinese-led copper mine project in central Myanmar.

Witnesses said dozens of monks and other protesters were injured when security forces used incendiary devices that set fire to protesters’ encampments outside the offices of the Chinese company, which has a partnership with the powerful military in Myanmar, formerly known as Burma.