The two opposing forces used poles and protest banners to poke at each other outside the township hall in Tacheng, Changhua county, in the hours before the hearing.
Police erected a barrier to separate the two groups, but the protesters broke through it early in the day.
By the time the hearing began on the project budgeted at 600 billion Taiwan dollars (20 billion US dollars), the brawl had petered out, with no injuries reported.
About 400 to 500 protesters were present, according to local police, and 200 officers were deployed.
The complex, proposed by partly state-owned Kuokuang Petrochemical Technology Co., has been under consideration since the 1990s.
Since then the proposed construction venue has been changed several times due to strong objection from residents.
According to the latest blueprint proposed by the company to the government for environmental impact evaluation, the proposed complex will occupy 2,836 hectares of coastal land.
The first phase is scheduled to be completed in 2016 and could help boost economic growth significantly, according to its backers.
But environmental activists insist that Taiwan, long plagued by industrial pollution, can no longer afford such large energy-guzzling projects.