Over 3,500 people who swam across the Pearl River, the third longest in China, have so far shown no symptoms of vomit or fever, local sources said.

They plunged into the river in the southern city of Guangzhou on Wednesday afternoon to show to the public that the once-polluted river is clean enough for a swim.

They considered the activity a beginning of a mass campaign to improve the environment and fight against water pollution.

However, some of the swimmers said that the water is still muddy. "Under the water, I could not see things half a meter in front of me. And my eyes were uncomfortable," said a swimmer whose surname is Fan.

Local media including the Guangzhou Daily and South China Metropolis Daily hailed the activity as a symbol of achievements the Chinese government has made in pollution control of major rivers.

Guangzhou has spent 8 billion yuan (US$1 billion) in building sewage treatment centers over the past three years.

The volume of the city's industrial waste water has decreased from 250 million tons in 2001 to 197 million tons in 2005.

The swimming pollution awareness campaign was organized by the municipal government of Guangzhou, capital of the southern province of Guangdong.

"We come here not for swimming," said Zhang Guangning, mayor of Guangzhou. "We hope the activity will make the local residents become aware of the importance of pollution control."

The 2,200-kilometer river is 400-700 meters wide as it runs through Guangzhou with a length of 80.6km.

The last mass crossing of the Pearl River was in the 1970s. Since then, it has been impossible for a large group of people to swim across it due to heavy pollution.

Huang Huahua, provincial governor of Guangdong, who also joined the mass swimming, said he believed the water is no longer thick and smelly. "Although there is still a long way to go to make it completely clean, we are confident that the water quality will become better and better," Huang said.

Local people used to fetch drinking water directly from the river, however, the accelerating industrialization in the Pearl River Delta in south China since the late 1970s resulted in serious pollution.

As factories discharged more and more waste water into the river, water began to smell badly, and fish could no longer live in the river.

"I often went swimming in the river when I was a child," recalled Liu Youhong, a Guangzhou-based environmentalist. "The water became so smelly that nobody dared to swim there and people even covered their noses when walking nearby," the 65-year-old man said.