SEOUL (AFP) - South Korea mounted a massive security operation Tuesday as US President George W Bush arrived for a two-day visit and opponents of American beef imports took to the streets. Bush touched down at a military airport south of Seoul and was driven to a luxury hotel after a welcoming ceremony including a 21-gun salute. A few miles away, drum-beating and flag-waving protesters streamed into a downtown plaza ringed by hundreds of riot police and police buses. Conservative activists earlier in the day had staged a far larger show of strength in support of the visit. Police said about 7,000 officers would guard Bush, while 17,000 more were being deployed during his stay to control the beef protests - the latest in a months-long series sparked by the supposed dangers of mad cow disease. Thousands of troops will also be mobilised, the Defence Ministry said. President Lee-Myung Bak, a pro-US conservative, ordered tight security. The close US relationship is "the backbone of South Korean diplomacy," he told his cabinet. After a decade of sometimes strained relations under liberal presidents, Lee has made stronger US ties his top foreign priority. But his first summit with Bush, at Camp David in April, ushered in a summer of discontent at home. His government's decision on the eve of the summit to resume US beef imports, in a bid to pave the way for a broader free trade agreement (FTA), led to months of occasionally violent rallies. The rallies largely subsided after Seoul secured extra health safeguards for US beef imports. Police estimated the Tuesday evening protest at the Chonggye plaza numbered only 2,000, while organisers put it at 5,000. "Down with Lee Myung-Bak" and "We oppose Bush's trip," demonstrators chanted. A student trampled on a picture of Bush and Lee bearing the slogan: "No Bush. No mad cow." Riot police briefly fired water cannon and detained about 30 people when protesters tried to occupy a street after marching from the plaza. Earlier Tuesday, some 30,000 military veterans, rightwing activists and conservative Christians prayed for a strong alliance, according to a police estimate. "Welcome President Bush " Let's strengthen US-Korea alliance" read a giant banner suspended from balloons. Hymns blared through loudspeakers. Meanwhile, ahead of his Beijing Olympics trip, Bush said he hoped China would strike a fine balance between maintaining security and respecting the spirit of the global event. He told the Washington Post in an interview published Tuesday that Beijing was overly sensitive over the terrorism threat in the run up to the games opening on Friday. "They're hypersensitive to a potential terrorist attack," Bush said in the interview aboard Air Force One taking him to the Asia trip that also covered South Korea and Thailand. "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," Bush said. Bush also said in the interview that it was "really hard to tell" whether human rights in China had improved during his administration despite his candid approach in dealing with the Chinese leaders on the sensitive issue. "I mean, this is a closed society in many ways," he said. "The Internet provides interesting opportunities for people to express themselves. Sometimes it's open, sometimes the filters are there. "I've talked to the evangelicals who go there who feel like the underground church movement has gotten a few steps forward, a step-and-a-half back. It's really hard to tell," Bush said.