Thousands of workers in South China's largest shoe company marched in protest in the city of Dongguan, Guangdong Province over contract and social security benefit issues.

Some 2,000 to 3,000 out of 40,000 striking workers took to the street in protest on Tuesday morning, said a worker who requested anonymity.

They marched from the factory to Gaobu town government building and staged a sit-in protest, he told the Global Times.

Some 1,000 to 2,000 workers of Yue Yuen, an original equipment manufacturer (OEM) company, protested in front of the factory gate Tuesday, confirmed an officer at Gaobu police station who requested anonymity.

Yue Yuen belongs to Taiwan-based Pou Chen Group, the biggest supplier in China that produces sneakers for brands including Nike and Adidas.

Adidas said in a statement earlier it was closely monitoring the situation while Nike did not comment on the strike.

There has been no violence so far, the officer said. The police have been asked to maintain order as strikes were called and canceled over the last 10 days. The strike broke out on April 5 over social security benefits, the officer said.

The workers protested in the morning and their representatives were back at the negotiating table with the employer by the afternoon, the police officer told the Global Times.

Pou Chen Group spokesman Ho Mingkun said it is in talks with government officials and labor representatives, the Taiwan-based Central News Agency reported.

The workers were unhappy the company did not pay social security or housing fund contributions based on their real salaries but the minimum amount instead, explained Zhang Zhirui, a legal consultant at a non-governmental labor dispute service in Shenzhen.

The company said it planned to raise the social security contribution in May as requested by workers, but many workers felt dissatisfied when their salaries dropped after deductions.

An employer must pay at least 11 percent and an employee 8 percent of their monthly salary toward social security insurance, according to Dongguan's labor regulations.

The workers also claim their contracts are different from common contracts used by Guangdong Province and therefore illegal. The company said the contract has been confirmed as legal and valid by the local authority.

With factories in China, Indonesia, Vietnam, the US and Mexico, Yue Yuen manufactures shoes for Reebok, ASICS, New Balance, Puma, Converse, Salomon and Timberland.

This isn't the first time Yue Yuen factory workers have had a strike. In 2011, 7,000 workers went on strike over layoffs and wage cuts.