By Robert Kozak

LIMA, Peru--Two days after President Ollanta Humala declared martial law in parts of Peru's southern state of Arequipa, the national police have detained protesters and reopened roads there that were blocked for weeks by protests against a copper-mine project known as Tia Maria.

The two-month state of emergency allows police to break up meetings and enter into homes without search warrants in those parts of Arequipa, where residents want Southern Copper Corp. (SCCO, SCCO.VL) to end the $1.4 billion project.

"Roads have been reopened and movement is no problem," a police spokeswoman said. Peru's military has also been guarding sites such as bridges and road tunnels.

Southern Copper declared a 60-day suspension of the project on May 15, but protests continued as farmers believe a mine will harm their water supplies in the Tambo Valley, about 650 miles south of Lima.

"The people are going to continue to resist any mine. They are stubborn. The people in Arequipa are that way," said Miguel Portugal, a social activist in the Tambo Valley region.

Antimine activists have announced they plan to hold protest marches in various parts of southern Peru on Wednesday and Thursday.

"We are taking precautions, and we hope that there won't be any major incidents, but we have to be prepared for any kind of aggression," Interior Minister Jose Perez Guadalupe was quoted as saying by government news agency, Andina.

Three protesters and one policeman have died in the clashes since March 23, when residents started blocking roads and bridges.

The protests have harmed President Humala's popularity, driving it down to 21%, pollsters say. A survey by consultant Apoyo Consultoria found that 94% of business leaders believe the government doesn't have a clear strategy for ending the conflict.

Protesters say a mine will harm farming by contaminating water from the Tambo River, used to irrigate rice, bananas and other crops. The company plans to pump desalinated water from the nearby Pacific ocean and says the mine won't harm agriculture.

Southern Copper, majority owned by Mexico's Grupo Mexico (GMEXICO.MX), plans to dig out 120,000 metric tons of copper a year starting in mid-2017.

"We regret the loss of lives, condemn all acts of violence and call for peace," a Southern Copper spokesman said Monday in an email.

Other disputes have caused delays in mining projects in recent years in Peru, including the $5 billion copper and gold-mine project known as Minas Conga, majority owned by Denver-based Newmont Mining Corp. (NEM).

Peru is one of the world's largest producers of gold, silver, copper, zinc and other minerals.

Write to Robert Kozak at robert.kozak@wsj.com

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