PHNOM PENH, Cambodia (AP) _ Cambodian police clashed Monday with workers at a factory that makes clothing for the U.S. sportswear company Nike, arresting seven, in the latest violence linked to a strike over salaries, union organizers and rights activists said.

The protest at the Taiwanese-owned Sabrina (Cambodia) Garment Manufacturing plant in Kompong Speu, just west of Phnom Penh, began May 21 when workers demanded that their salaries be raised to the equivalent of $88 per month from $74. Their net pay is calculated to include overtime and various allowances, and there is a dispute over whether they are receiving the legal minimum wage.

Free Trade Union of Workers of the Kingdom of Cambodia organizer Son Vanny said its members exchanged barrages of sticks and stones Monday with members of a rival union opposing the strike, after which riot police arrived to break up the crowd. He did not identify the other union, though there are several in Cambodia and those aligned with the government are less militant.

The strike has seen violence before. Police allegedly used force when workers blocked the main road outside the factory on May 27, reportedly causing two pregnant workers to miscarry. Nike, for whom the factory is a contractor, expressed concern after that incident. The factory employs more than 3,000 workers, mostly females.

Heng Sophors, an activist with the local human rights group Licadho who monitored the strike Monday, said more than 1,000 riot police with batons and shields were deployed around the factory. He said some police were the targets of sticks and stones thrown by the protesters as they moved in to break up the crowd. He added that some strikers were slightly injured by police.

Son Vanny accused police of beating and detaining only workers from his union. He said police alleged the workers had incited violence, a charge he denied.

Son Vanny and another union official, Thorn Thon, said seven workers were arrested. Police at the scene and at the Interior Ministry declined to comment on any arrests or injuries.

The garment industry is Cambodia’s biggest export earner, employing about 500,000 people in more than 500 garment and shoe factories. In 2012, the Southeast Asian country shipped more $4 billion worth of products to the United States and Europe.

Pay is low and working conditions are usually uncomfortable at best. Last month, the ceiling of a Cambodian footwear factory collapsed, killing two people and injuring seven.

Cambodia hosts a unique program of the International Labor Organization called “Better Factories Cambodia” that assesses and reports on working conditions in all the country’s export garment factories. The impetus for the program was an agreement under which Cambodia pledged to improve labor conditions in exchange for better trade privileges with the United States.

Because the industry is Cambodia’s economic locomotive, garment workers also carry some political clout, a special concern for Prime Minister Hun Sen’s government because it is an election year and some unions have long-standing ties with the political opposition.