Citing critics including government experts and advisers, they said the factory, next to a residential area, was a "timebomb" for public health and a grave threat to Xiamen's seaside environment.

Police detined Li Yiqiang on June 3 and issued an arrest warrant to his family a month later on charges of illegal assembly and organising marches, Zhang Likun, a Beijing-based friend, told Reuters by telephone.

Zhang said other protesters may also been detained.

Li, 39, rose to prominence pressuring the government to assert its claim of sovereignty over a group of disputed islands in the East China Sea. The Japanese-controlled islands are claimed by China as Diaoyu and by Japan as Senkaku.

Li in past years has set sail for the islands on three separate occasions, said Zhang, a fellow Diaoyu activist.

Zhang said police had video footages of Li making speeches during the marches against the chemical plant.

Tong Zeng, another friend and fellow Diaoyu activist, said the charges against Li were unfair and argued that it was a public order offence at most.

"He had neither the intention nor the ability to organise such a huge protest," Tong told Reuters. "I guess he was just a bit active during the march and his beard made him stand out."

Both men learned about Li's arrest through his sister, Li Yan, who could not be reached on Thursday.

Xiamen police declined to comment when reached by telephone.

Pollution alongside breakneck economic and industrial growth has become an increasingly inflammatory issue, galvanising normally apolitical urban residents into collective action.

Stand-offs -- in some cases violent -- between local governments eager to push big projects for growth and tax revenues and residents who want clean water, air or a quiet environment have been on the rise.

China's constitution grants citizens the right to stage demonstrations, but police rarely approve protests, which are seen by the stability-obsessed Communist Party as sensitive.

The Xiamen city government has suspended construction of the factory to conduct further impact assessment, but it has also accused "hostile forces with ulterior motives" of masterminding the June marches.