Riot police forcefully dispersed about 500 demonstrators early Sunday morning in central Seoul after an all-night protest against the impending resumption of U.S. beef imports.

The diehard protesters were part of a crowd of about 40,000 South Koreans who had gathered in front of Seoul City Hall Saturday night to protest a beef import agreement with the U.S. -- the largest rally since candlelight vigils started early May.

No serious injuries were reported. But police said they hauled away over 200 protesters, raising the total number of detainees since the start of the protests to 500.

The rally was mostly peaceful, with most protesters waving candles and dispersing voluntarily near midnight. But hundreds of participants demonstrated on downtown streets after midnight.

Police fired water cannons -- for the first time since the protests against U.S. beef began -- as scores of protesters attempted to head for the presidential office. Several protesters injured slightly by the water cannons were taken to hospital.

Anger has intensified since Thursday when the government started a legal process to put the new import terms into effect. Imports are expected to resume as early as this month.

Rallies in recent days have ended with protesters marching through downtown streets, resulting in some scuffles with police. Over 10,000 riot police have been dispatched daily to prevent possible turmoil.

Opposition legislators have also been taking to the streets to join the protests, accusing the government of striking a hasty deal with Washington for the sake of better economic ties between the two countries. The ongoing political tension over beef imports is expected to cripple to first session of South Korea's new
legislature which opened Friday.

Suspecting political forces are behind the protests, the government has been investigating the leaders of five civic groups that organized the candlelight vigils.

But citizens insist that their attendance is voluntary and reflects overall public discontent with the beef deal and the Lee administration.