The crackdown was conducted by security forces who have little experience using modern crowd-control methods that are meant to minimize casualties. During military rule, dissent was brutally repressed and protesters on several occasions were shot to death in the streets.

In Thursday’s raid, security forces fired what one witness described to The Associated Press as “black balls that exploded into fire” to disperse the protesters, who had overstayed a Wednesday deadline set by the government to leave the area.

Factory workers and villagers, both ethnic Burmese and members of minorities, have taken advantage of new freedom under Mr. Thein Sein’s government to carry out limited demonstrations and strikes in recent months. The protests at the copper mine were by far the largest to appear since the former military junta ceded power to civilians in March 2011.

By dealing so forcefully with the mine protests, the government risks appearing to defend the vested interests of the old government. The project is typical of the kinds of opaque deals often struck during military rule that have enriched many of the country’s generals. The military has been so deeply involved in business that it has its own company, the Union of Myanmar Economic Holdings, that is listed as a part owner of the copper mine.

The deal between the military and the Chinese company, a subsidiary of a state-owned Chinese arms manufacturer, to expand the mine was signed two years ago when Mr. Thein Sein was prime minister under the military junta.

According to an American 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 government, especially the country’s former dictator, Senior Gen. Than Shwe.