SEOUL, South Korea - August 5, 2008 - Police fired water cannons at thousands of protesters Tuesday as President Bush got a volatile reception in South Korea at the start of his three-nation Asian trip.

Dueling demonstrations reflected mixed sentiments in this U.S. ally, where public opinion surveys remain generally positive about America, though many people decry Washington for a variety of issues. Bush will meet Wednesday with President Lee Myung-bak for the third time since the conservative, pro-American leader took office in February.

Some 18,300 police were on high alert with riot gear and bomb-sniffing dogs to maintain order during Bush's brief visit, the National Police Agency said.

About 30,000 people gathered in front of Seoul City Hall for an afternoon Christian prayer service supporting Bush's trip. Large South Korean and U.S. flags were held aloft by balloons overhead along with a banner reading, "Welcome President Bush."

As evening approached, an estimated 20,000 anti-Bush protesters gathered nearby. Police turned water cannons on them as they tried to move onto the main central downtown boulevard, telling the crowd that the liquid contained markers to tag them so they could be identified later.

"I don't have anti-U.S. sentiment. I'm just anti-Bush and anti-Lee Myung-bak," said Uhm Ki-woong, 36, a businessman who was wearing a mask and hat like other demonstrators in an apparent attempt to conceal his identity.

The anti-Bush crowd dwindled later in the evening to several thousand people, with the hard-core remnants turning aggressive. Protesters shattered the windows of a police bus and authorities responded by again firing water cannons.

About 70 demonstrators were arrested, police said, in addition to another 12 near the military airport where Bush landed.