An independent investigation into a crackdown on a copper mine protest in Myanmar that left dozens injured has found that phosphorus was used by security forces, according to a group of lawyers.

A canister found at the crackdown site was sent to a laboratory overseas for tests, said Thein Than Oo, a member of the network of lawyers that conducted the probe.

"The result said it contained 25 percent phosphorus," he told AFP, speaking on Thursday. "It can cause serious injuries. It's for military use not for civilian forces."

It was not possible to verify the claim and there was no immediately comment from the authorities.

The group of lawyers said it had sent the findings to opposition leader Aung San Suu Kyi, who is heading a separate parliamentary probe into the issue.

The pre-dawn raid on protest camps at the Chinese-backed mine in Monywa in northern Myanmar in November was the toughest clampdown on demonstrators since a reformist government came to power in early 2011.

Activists said about 100 people were injured in the crackdown. Some suffered severe burns.

The government has since apologised to senior Buddhist clerics over injuries sustained by monks who were at the forefront of the protest.

President Thein Sein's office initially said that tear gas and smoke bombs were used against the protesters, but denied allegations by local media that a form of chemical weapon was deployed.

Chinese-backed projects to tap Myanmar's abundant natural resources have sparked resentment among local residents, who have been testing the new reformist government's proclaimed tolerance of freedom of expression.

Opponents are calling for work at the mine -- a joint venture between Chinese firm Wanbao and military-owned Myanmar Economic Holdings -- to be suspended to allow environmental and social impact studies.

On Tuesday more than 1,000 people, including local villagers, marched to Wanbao's offices in Monywa in a protest to call for answers two months after the crackdown.

About 400 police were deployed but the rally ended peacefully, according to activist Ye Yint Kyaw.

"We just want to raise attention before the investigation commission releases its report. We will wait and see whether the commission works in the interests of the people or not," Ye Yint Kyaw said.