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.
Photos from online Burmese news sites showed monks, who are highly revered in the country and often involved in political causes, with singed saffron robes stuck to their badly seared skin.
The raid came hours before a scheduled visit to the area by Aung San Suu Kyi, the Nobel laureate and leader of the opposition in Parliament. The trip by such a high-profile figure as Suu Kyi to the city of Monywa, near the mine, underlined the support and resonance that the protests had across the country before Thursday’s raid.
Analysts said the brutal way that the crackdown was carried out could hurt the popularity of Thein Sein as he tries to convince the country that his government has made a clean break from the military regime that ruled for five decades.
‘‘There will be political consequences,’’ said U Thiha Saw, the editor of Open News Journal and Myanma Danna magazine. ‘‘This may be the start of an uglier phase for the government. Things may get a little more complicated.’’
The crackdown was conducted by security forces inexperienced in modern crowd control. During five decades of military rule, which ended early last year, dissent was brutally repressed and on several occasions protesters were shot dead in the streets.
On Thursday, the security forces fired what one witness described as ‘‘black balls that exploded into fire.’’ The authorities had set a Wednesday deadline for the dispersal of protesters.
Factory workers, villagers, and ethnic minorities have taken advantage of new freedoms under Thein Sein’s government and carried out limited demonstrations and strikes in recent months. But protests at the copper mine were by far the largest since the military junta ceded power to the civilian government in March 2011.
By dealing so forcefully with the protests, the government risks being seen as defending the vested interests of the old regime. The project was typical of the opaque deals that took place during military rule — and which made many generals rich. The military has been so deeply involved in businesses that it has its own holding company, the Union of Myanmar Economic Holdings, which is listed as a part owner of the copper mine.
The deal between Myanmar’s military and the Chinese company, a subsidiary of a state-owned Chinese arms manufacturer, was signed two years ago when Thein Sein was prime minister in the military junta.
According to a US diplomatic cable made public by WikiLeaks, the deal was brokered by U Tay Za, a tycoon who became rich through his connections to the military regime, especially the country’s former dictator, Senior General Than Shwe.
Thursday’s crackdown also complicates the investment picture for China, which has struggled with the perception in Myanmar that it is mainly interested in extracting natural resources from the country.
The Global Times, a state-owned Chinese newspaper, published an article Thursday before the crackdown that accused the West and advocacy groups of instigating the protests and said shutting down the mine would be ‘‘a lose-lose situation’’ for China and Myanmar.
‘‘Chinese companies’ investments in Myanmar are facing huge challenges,’’ the article said. ‘‘What we see in the country is the inevitable impact of its democratization.’’
Anti-Chinese sentiment was a major factor in the cancellation of a hydroelectric dam project last year in northern Myanmar that would have exported electricity to China. The project was suspended after an outcry.
Thursday’s raid came in the predawn hours. Ashin Visara, a 28-year-old monk who was injured in the crackdown, said security forces threw ‘‘explosive devices’’ into the areas where protesters were camped out.
‘‘That started fires at the protest sites,’’ he said. ‘‘And then they attacked us.’’
U Nway Oo, a student activist from Monywa who assisted the injured, said many protesters fled into the surrounding villages or the jungle.
“There were no medical personnel or ambulances around before the crackdown,’’ he said.