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."

"The United States made sacrifices for South Korea during the Korean War and helped us live well," said Kim Jung-kwang, a 67-year-old retired air force colonel who wore his military uniform to the rally. "The United States is not our enemy. Without the U.S., we will die."

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.

Bush held off on visiting Seoul earlier this year when protesters staged nightly candlelight vigils and repeatedly clashed with riot police over imports of American beef, saying Lee ignored public health concerns over the possibility of mad cow disease and failed to consult with citizens. Lee has promised to patch up relations with Washington that became strained under Seoul's previous decade of liberal governments.

Bush calls Lee a friend, which is good considering the raft of sensitive topics they will tackle before the American president heads to Thailand, then to the Beijing Olympics.

At the top of the list is getting North Korea to live up to its commitment to dismantle its nuclear weapons program.

Sunday is the earliest that Washington could move to strike North Korea from a list of state-sponsors of terrorism, a long-held demand from Pyongyang. But first, Washington wants the North to agree to procedures for verifying a declaration of its nuclear programs that Pyongyang submitted to the international arms talks - six months late and with fewer details than the U.S. originally demanded.

Washington has called for North Korea to allow thorough inspections and interviews with nuclear scientists, but Pyongyang has so far not accepted the proposal.

"We're at a very critical moment now for the North Korean government to make a decision as to whether or not they're going to verify what they said they would do," Bush said in an interview with China's state-run CCTV last week. "It's one thing to say it, but I think it's going to be very important for them to understand that we expect them to show us."

Grateful for South Korea's troop contribution in Iraq, Bush also will try to persuade Lee to make a bigger contribution in Afghanistan to help deal with the Taliban's resurgence.

"Obviously we'd like to see a greater role for South Koreans in Afghanistan, if the South Korean people are willing to move in that direction," Dennis Wilder, the National Security Council's senior director for Asian affairs, told reporters on Air Force One.

Also on the agenda will be efforts by both presidents to have their legislatures approve a free trade agreement, with estimates it could increase bilateral trade by 25 percent. But with free trade deals with Colombia and Panama stalled in Congress, the prospects for ratification by the end of the year are unlikely.

Bush will meet with U.S. troops based in South Korea, as he did during a stopover in Alaska, where he expressed gratitude for their role in fighting terrorism.

"About a year ago, people thought Iraq was lost and hopeless," Bush said at Eielson Air Force Base, where he posed for photos with airmen and soldiers and worked the crowd, at one point lifting a baby in the air. "People were saying, 'Let's get out of there, it doesn't matter to our national security.'

"Iraq has changed - a lot - thanks to the bravery of people in this hangar and the bravery of troops all across our country. The terrorists (are) on the run. The terrorists will be denied a safe haven, and freedom is on the march. And as a result, our children are more likely to grow up in a peaceful world."

Bush's Asia trip also includes stops in Thailand and China. In an interview aboard Air Force One with The Washington Post, Bush said it was "really hard to tell" whether human rights in China had improved over the past eight years.

Bush said he speaks candidly with Chinese President Hu Jintao about human rights, but he skirted a question about a pre-Olympics security drive by Chinese authorities.

"They're hypersensitive to a potential terrorist attack," Bush said in the article for Tuesday's editions of the paper. "And my hope is, of course, that as they have their security in place, that they're mindful of the spirit of the Games, and that if there is a provocation, they handle it in a responsible way without violence."

This video was broadcast on August 5, 2008.