SHANGHAI/CHICAGO (Reuters) - Workers striking at Wal-Mart Stores Inc <WMT.N> outlets in China have returned to work after the firm agreed to consider their protests against a new work scheduling system that some fear could be used to cut overtime pay, workers and labor activists say.

The strikes started at the beginning of the month in the southern city of Nanchang and spread with the help of social media to Wal-Mart hypermarkets in two other cities, the activists and workers said.

Worker unrest has surged in China as slowing economic growth and high costs have squeezed companies, but it is relatively rare for workers to organize across provincial lines.

Wal-Mart said on Thursday it had introduced the new work hour scheduling system in July across its hypermarkets in China, and a majority of its employees supported it.

But Duan Yu, a worker at Wal-Mart in Nanchang who was representing the strikers in discussions with management, said employees had been voicing their displeasure about the system since May to no effect.

"I have a very bad feeling about it. There's no possibility they'll agree (to our demands)," Duan said by telephone.

Wal-Mart management at Duan's store in Nanchang called a meeting of all workers on Thursday, but afterward tried to break them up and speak to them one by one to try to "find people with sympathetic ears", Duan said.

China Labour Bulletin, a Hong Kong-based workers' rights group, said the strikes had ended because Wal-Mart had agreed to respond to the workers' demands within a week.

Wal-Mart China declined to comment on whether it had agreed to talks with the workers.

Discontentment at the scheduling system spread fast via a string of online chatrooms on Tencent Holdings Ltd's <0700.HK> messaging app WeChat under the loose organization of an online group called "Walmart Chinese Workers' Association".

About 50-60 workers at each of four stores in Nanchang, Chengdu and Harbin had been involved in the strikes, said Zhang Liya, a Wal-Mart employee from the southern city of Shenzhen who set up and manages the online group.

"(The stores) launched the strikes on their own," said Zhang. "If Wal-Mart malevolently continues with this then there will definitely be even more stores that stand up."

U.S.-based Wal-Mart spokeswoman Jo Newbould said the hour scheduling system, which is unique to China, gives workers the flexibility to work additional shifts if they want to.

"We have communicated the new system to Wal-Mart China associates and the majority of associates support it," she told Reuters.

(This version of the story was refiled to amend translation of name of organization in paragraph 10)

(Reporting by John Ruwitch in SHANGHAI, Nandita Bose in CHICAGO and Jake Spring in BEIJING; Editing by Leslie Adler and Will Waterman)