The crackdown is a big blot on the government's efforts to woo popular support, especially because many of the targets were monks, who are admired for their social activism as much as they are revered for their spiritual beliefs in deeply religious Myanmar.
The previous military government infamously cracked down violently on monks who were leading the 2007 pro-democracy protests that came to be known as the "Saffron Revolution," from the color of their robes.
Monks in Myanmar's two biggest cities, Yangon and Mandalay, staged small nonviolent protests Friday.
The Upper Myanmar Monks organization in Mandalay issued a statement calling on the government to formally apologize for the action within five days, to provide sufficient health care for those who were injured and to release seven monks they say were detained.
U Withuta, a prominent activist monk who is a member of the group, said more than 40 monks were hurt, some seriously and at risk of losing their eyesight. He said he was lightly burned on the thigh.
"We wanted to forget what happened in 2007 and proceed forward, but what happened yesterday was like opening an old wound," Withuta said. He said it was premature to say what the monks would do if their demands were not met.
Citizen activism has increased since the elected government took over last year. Street demonstrations have been legalized, and are generally tolerated, though detentions have occurred in sensitive cases.
Political and economic liberalization under Thein Sein has won praise from Western governments, which have eased sanctions imposed on the previous military government because of its poor record on human and civil rights.
The Letpadaung mine is a joint venture between China's Wanbao Mining Copper Ltd. and the military-owned Union of Myanmar Economic Holdings Ltd. Many in Myanmar remain suspicious of the military and see China as an aggressive and exploitive investor that helped support its rule.
In Yangon, more than 30 monks who staged a peaceful protest at downtown Sule pagoda, were joined by nearly 100 people who chanted prayers in front of the office of the army's holding company.
"May all be free from harm, may all be peaceful and may the Letpadaung mountains be green," they chanted in the Friday dusk.
Associated Press writer Matthew Pennington in Washington contributed to this report.